Lemon Cake with Lemon Curd and Swiss Buttercream

This lemon cake features a moist, light, lemon sponge cake with a tangy lemon curd filling and a light and creamy Swiss buttercream frosting! It's bright, vibrant, and perfect for spring! 

This lemon cake is guaranteed to light up your taste buds in the best way possible! The cake is made with fresh lemon juice and is incredibly moist and light. The frosting is a cross between Swiss meringue buttercream (SMBC) and cream cheese frosting. There's also a tangy lemon curd filling which is unbelievably fresh, vibrant, and seriously addicting! 

If you've never heard of SMBC, it's a basic frosting that is made with egg whites that are gently cooked over a double boiler and then whipped to stiff, glossy peaks. It's perfect for frosting cakes because it's smooth, light, and spreads or pipes easily. It's also not overly sweet or rich like American buttercream can be. Adding some cream cheese to the frosting makes it a little thicker and adds a delicious, tangy flavor.  

What You Need to Make the Lemon Cake:

• All-purpose flour: Measure correctly. It's so easy to over-measure flour and end up with a cake that's dense instead of light and fluffy. Stir the flour around in the bag to loosen it up, then lightly spoon it into your measuring cup and level it off with the flat side of a knife. 
• Baking powder and baking soda: Leaveners that help the cake rise and make it soft.
• Salt: A full teaspoon brings out and enhances the cake's flavor. Make sure you're using unsalted butter or the cake will be way too salty. If all you have is salted butter then reduce the added salt to half a teaspoon. 
• Butter: Unsalted and softened to room temperature. Leave it out on the counter for at least 1 hour (or longer if your house is pretty chilly). It should be firm but easily pliable when gently pressed in. You can also speed up softening by cutting the butter into small pieces. 
• Sugar: Creaming the butter with granulated sugar whips air into the butter and helps make the cake light and airy!
• Eggs (room temperature): Room temperature eggs mix into the batter more easily and evenly. Take them out of the fridge at least 1 hour before using them. You can also quickly bring the eggs to room temperature by placing them in a bowl of lukewarm water for 5-10 minutes. 
• Lemon juice: Fresh-squeezed, never bottled!  
• Sour cream (room temperature): Sour cream is the ultimate insurance policy for a perfectly moist cake. Always use full-fat sour cream! You can also substitute with full-fat plain Greek yogurt. 
• Whole milk (room temperature): If the milk is too cold when you add it in it can make the batter turn lumpy. It's okay if that happens though, the batter will smooth out once the dry ingredients are mixed in.

How to Make Lemon Cake:

1. Whisk the dry ingredients in a medium bowl and set aside.
2. In a large bowl or a stand mixer, use a hand-held electric mixer or the paddle attachment in a stand mixer to cream the softened butter and the sugar together until light and fluffy. Add the room temperature eggs, one at a time, and mix until just combined after each. Don't overmix.
3. Add the vanilla extract, fresh lemon juice, sour cream, and milk. Mix until combined and use a rubber spatula to scrape down the sides of the bowl. 
4. Add the dry ingredients and mix until smooth.
5. Divide the batter evenly between the three cake pans, a kitchen scale is best for making sure the batter is divided evenly. Tap the pans on the counter to spread the batter evenly in the pan and to get rid of any trapped air bubbles. Bake in a 350° oven for 22-25 minutes, or until a toothpick inserted into the center comes out clean.
6. Remove the cakes from the oven and let cool slightly in the pan, then cool them completely in the fridge or freezer before frosting. Allow the cakes to cool slightly in the pan and then cool completely in the fridge or freezer before frosting, overnight is best if you can. You can also wrap the cakes in plastic wrap and freeze them for up to 3 months. 

Lemon Curd Filling: 

• Eggs: 4 large egg yolks. Separate the eggs while they're cold and keep the whites in a separate bowl for making the frosting. 
• Sugar: A full cup of sugar is perfect for balancing out the tart lemon flavor! 
• Salt & vanilla extract: Just a small amount to add flavor. 
• Fresh-squeezed lemon juice: You need half a cup of fresh-squeezed lemon juice. 3 or 4 large lemons should be enough, but it's always better to have a few extra just in case. You can always use any leftovers for decorating the cake! 
• Flour: For thickening the lemon curd.  
• Butter: Salted or unsalted is fine since you're only using a couple of tablespoons.

Make the Lemon Curd: 

1. Whisk the egg yolks, granulated sugar, vanilla extract, and salt in a medium bowl.
2. In a separate bowl, whisk the fresh-squeezed lemon juice and flour until the flour is dissolved.
3. Melt the butter in a medium saucepan, add the lemon juice/flour mixture and cook until thick. Whisking constantly. 
4. Slowly add the hot lemon mixture to the bowl with the egg yolks, whisking the yolks constantly as you do. Return everything back to the saucepan and bring the mixture to a boil, whisking constantly until very thick. About 6-8 minutes. 
5. Remove from heat and use a sieve to strain the lemon curd into a small bowl. Straining the curd helps get rid of any lumps of flour. Use a whisk to help pass it through the sieve and use a spoon to scrape the strained curd from the opposite side and into the bowl. Cover the bowl with plastic wrap and keep it in the fridge to set slightly while you make the frosting.  

For the Frosting:

• 4 large egg whites
• 1 and ½ cups granulated sugar
• 1 cup unsalted butter, softened to room temperature, and cut into 1" pieces.
• ¼ tsp. salt
• 1 tsp. vanilla extract
• 2 (8 oz.) packages cream cheese, softened to room temperature. 

How to Make SMBC Frosting:

1. Bring an inch of water to a gentle boil in a medium pot. Whisk the egg whites and the granulated sugar in a bowl large enough to fit over the pot without the bottom of the bowl touching the water. 
2. Place the bowl over the pot of gently boiling water and cook, whisking constantly, until the sugar is dissolved and the temperature reaches 165°. 5-6 minutes. Remove the bowl from the pot and transfer the contents to a large, clean bowl or bowl of a stand mixer fitted with a whisk attachment. 
3. Use a hand-held mixer or the whisk attachment at medium speed to whip the mixture to stiff glossy peaks. About 10 minutes, it should be thick and glossy with a marshmallowy texture and appearance. 
4. Keep the mixer running at medium speed and add the softened butter, 1 piece at a time, and mix until fully incorporated after each. Continue until all of the butter is added in. The frosting should be thick, glossy, and smooth. Add the salt and vanilla extract and mix until smooth. Add the softened cream cheese and mix until smooth. 

Tips for Making the Frosting:

• Separate the eggs while they're cold and make sure there aren't any traces of egg yolk in the bowl or you won't be able to whip it to stiff peaks. If you accidentally get some yolk in the bowl you can use an eggshell to scoop it out or just start over if you can't get all of it out.
• It's easier to whip the egg whites to stiff peaks if the bowl isn't hot, which is why I recommend transferring the mixture to a separate bowl instead of leaving it in the same bowl you cooked the egg whites in. If you want to leave it in the same bowl, you can cool it down by surrounding the bowl with bags of ice or some frozen vegetables. 

How to Fix a Soupy SMBC:

After you've added the butter to the frosting it should be smooth, thick, and glossy. If it's soupy, then your butter was too soft when you added it in. To fix it, just set the bowl in the fridge for about 30 minutes or until it firms up. You can also refrigerate SMBC overnight, cover the bowl tightly with plastic wrap and give it a quick whip once you're ready to use it.

How to Fix a Broken SMBC:

The frosting should be smooth and not at all grainy. If it looks grainy or curdled then you let the frosting get too cold or the butter was too cold when it was added in. To fix it, place the bowl back over a pot of gently simmering water and whisk until smooth and glossy. You can also microwave the meringue in 15-second intervals, whisking in between, until smooth. If you take it too far and it becomes soupy, just place the bowl in the fridge until it firms back up. Easy! 

How to Frost Lemon Cake:

1. Once the cakes have cooled completely, use a large serrated knife to level off the domed tops.
2. Place one of the cakes, cut side up, onto a cake stand or a cake board. Add the frosting to a piping bag and pipe a thick border of frosting around the outside of the cake. Spread some lemon curd inside the border of frosting, if it isn't spreading easily, you can add the lemon curd to a piping bag and simply pipe it around inside the border of frosting. Use a spatula or the piping bag to top the layer of lemon curd with more frosting. 

3. Place the second cake layer with the cut side down on top of the first cake layer. Repeat with another border of frosting, more lemon curd, and top the curd with more frosting. Place the final cake layer on top with the cut side down and repeat with the border/curd/frosting process again. 

4. Use an offset spatula to spread frosting all over the top and around the sides of the cake. The first coat is called the crumb coat, it's okay if it's not pretty. Place the cake in the fridge for at least 30 minutes before adding a second coat. Use an offset spatula or a cake scraper to spread it evenly on top of the cake and around the sides. 

Tips for Making Lemon Cake:

• It's always easier to frost a cake when it's very cold. I like to freeze the cakes in the pan overnight so they stay cold while I'm frosting them.
• Because of the way heat is distributed in your oven, it's best to only bake cakes 2 at a time. If you want to bake them all at once, you might need to rotate the pans in the oven about halfway through baking so they bake evenly. 
• For cakes with flat, even layers, try using cake strips! They help the cakes bake more slowly and evenly so you don't have to level off the tops, which makes frosting a lot easier and way less messy! You can buy a set on Amazon or make your own DIY version with some wet paper towels or newspaper wrapped in aluminum foil.

More Cake Recipes:

• Berry Cream Cake
• Chocolate Bundt Cake
• Red Velvet Cake
• Coffee Cake
• Carrot Cake