I recommend making the lemon curd the night before you make the cake so that it has time to fully cool.
Create a double boiler by bringing 1-2 inches of water to a simmer in a small saucepan. Place a heatproof glass bowl on top and make sure it's not touching the water.
Add the egg yolks, sugar, lemon juice, lemon zest, and salt into the top of your double boiler. Whisk until completely combined and then continue to whisk as the lemon curd cooks (constant whisking prevents the lemon curd from curdling). Continue to whisk the lemon curd for 10-15 minutes or until it become thick and paler in color (it should be similar to the consistency of hollandaise sauce). If the lemon curd isn’t thickening, turn up the heat and constantly whisk.
Once thickened, remove the glass bowl from the heat. Cut the butter into chunks and whisk into the lemon curd until fully combined and smooth. If desire, pass through fine mesh strainer and then pour into a glass jar. Let cool to room temperature and then cover and store in the fridge.
Preheat the oven to 350°F and lightly spray three 6 inch layer cake pans with pan spray.
Whisk the cake flour, baking powder, baking soda, and salt together in a large bowl.
In a separate bowl, whisk the melted butter, sugar, eggs, vanilla, lemon extract, and lemon zest together until combined.
Stir in about half of the flour mixture. Then stir in the milk and the rest of the flour mixture and mix until fully combined. The batter will be thin.
Pour ⅓ of the batter into each cake pan (they will only be about half full). Bake at 350°F for 20-30 minutes or until a cake tester comes out clean.
Place a cooling rack on top of each cake pan and invert. The pan should slide cleanly away.
Let the cakes cool completely to room temperature before trimming.
Once the cakes are fully cooled. Using a sharp bread knife, trim the cakes until the layers are even. Wrap each layer tightly in plastic wrap and place in the freezer overnight.
Swiss Meringue Buttercream
In the bowl of a stand mixer, whisk the sugar into the egg whites then set the bowl over a saucepan filled with about two inches of simmering water over medium heat. Do not let the bottom of the egg whites bowl touch the water. Whisk the whites and sugar constantly until sugar is dissolved and mixture has thinned out (about 3-4 minutes).
Place the bowl on the stand mixer and fit with the whisk attachment. On medium-high speed, beat the mixture until stiff peaks form and the meringue is no longer warm to the touch, at least 10-15 minutes. If it’s still not reaching stiff peaks, stop the mixer, place the bowl in the fridge for 10 minutes, then return to the mixer and continue beating until stiff peaks form.
If the bowl and meringue still feel warm, wait until both cool to room temperature before adding the butter in the next step (or else the butter will melt). You can place the bowl in the fridge to speed up cooling.
Switch the stand mixer to the paddle attachment. On medium-high speed, add the butter 1 tablespoon at a time. Wait for the butter to fully mix in before adding the next tablespoon. After all the butter has been added, turn the mixer down to medium speed and fully beat in the lavender extract and salt for about 30 seconds.
The buttercream should be thick, creamy, and smooth. If your mixture has become too thin and soupy after you add the butter, place the entire bowl in the refrigerator for 20 minutes to cool down, then return it to the mixer and beat on medium-high speed until thickened.
Assemble the Cake
Remove cake layers from the freezer. Place the bottom layer of cake on your cake stand and spread a thin layer of buttercream over the top. Then pipe a thick border of buttercream around the edge. Spoon lemon curd evenly over the middle (about ¼ cup). Place the next layer on top and repeat.
Place the final cake layer upside down so that the top of your cake is as even as possible. Smooth the rest of the buttercream on the top and sides of your cake as desired. Use buttercream to attach candied lemon slices to the side of the cake.
Refrigerate your cake for 2-3 hours before slicing and serving.
This recipe is designed for 3 six inch cake pans. It will also work with 2 eight inch cake pans. Cover and store the cake at room temperature for up to 2 days or in the fridge for up to a week.Freeze any leftover buttercream for up to 3 months, just place in the fridge overnight to thaw.Store any leftover lemon curd at room temperature for up to a week or use to make lemon poppy seed macarons!