4 lemons (or 6 Meyer lemons)
2 whole eggs plus 4 egg yolks
1 cup sugar
4 tablespoons unsalted butter, at room temperature, cut into small even cubes
2 cups plus 6 tablespoons all-purpose flour
2 teaspoons baking powder
1 teaspoon salt
3 cups fresh blueberries
1/2 cup sour cream
1/2 cup whole milk
2 teaspoons pure vanilla extract
1/2 teaspoon pure lemon extract
1 teaspoon lemon zest
1 cup (2 sticks) unsalted butter, room temperature
1 and 1/2 cups sugar
4 eggs, room temperature
1 cup (2 sticks) unsalted butter, softened
2 teaspoons lemon zest
5 cups confectioners' sugar, sifted
2 tablespoons fresh lemon juice
1/4 cup whipping cream
1/4 teaspoon pure lemon extract
pinch of salt
yellow food gel coloring
make lemon curd: Wash citrus well and using a zester, remove all of the colored portion of the peel (not the white part - it's bitter) into a bowl. Set aside. Slice the citrus in half crosswise, and extract as much juice as you can using a citrus reamer. I like to juice it straight over a sieve set over a glass measuring cup so it strains and measures it all in one motion. Repeat until you have 2/3 cup strained juice. Get your double boiler ready by filling a saucepan with 1 inch of water, then placing a metal bowl on top of the saucepan. Ensure that the bowl fits snugly into the top of the saucepan and that it doesn't touch the water. Whisk the juice, whole eggs, egg yolks and sugar in the bowl until smooth. Add the butter cubes, but don't stir. Heat the water in the saucepan over low heat until it simmers and place the bowl atop the rim. Stirring gently, but constantly, using a heatproof spatula or wooden spoon, cook until the curd has thickened and all the butter has melted and is incorporated, about 10 minutes. To test if the curd is thick enough, remove the spatula or spoon from the curd and check that it's coated. Strain the curd over a bowl using a sieve and then stir in zest. Cover with plastic wrap pressed directly against the curd (to prevent a skin from forming) and chill for at least 3 hours (I like to chill it overnight). It will thicken more while chilling.
make cake: Preheat oven to 350 degrees. Prepare three 8-inch round cake pans with parchment paper, butter, and flour. combine dry ingredients in a bowl. Transfer 1 tablespoon flour mixture to a larger bowl and add blueberries. Toss to coat (this will prevent the blueberries from sinking to the bottom of the cake). Stir sour cream, whole milk, vanilla extract, lemon extract and lemon zest in a small bowl. In the bowl of a stand mixer, cream butter and sugar until pale and fluffy, about 5 minutes. Beat in eggs one at a time, until well combined. Add flour mixture in 3 additions, alternating with 2 additions milk mixture (ending with flour mixture). Stir just until combined. Add 1/3 of the batter to one cake pan. Gently fold blueberries into remaining batter. Divide batter among remaining two pans.
Bake cakes 25 minutes, until toothpicks inserted in center comes out clean. Cool cakes in pans on racks for about 10 minutes, then invert onto racks to cool completely.
make frosting: Cream butter and lemon zest in bowl of stand mixer with paddle attachment, about 3 minutes. Slowly add confectioners' sugar, mixing on low speed for about 2 minutes. Add remaining ingredients and beat on medium-high for 3-4 minutes until very fluffy. Add a drop or two of coloring gel, then beat until blended.
Spread a dollop of frosting to a cake board of cake stand. Transfer 1 blueberry cake, face up, to plate. Create a dam around the perimeter of the layer using frosting in a pastry bag fitted with a plain tip. Spread lemon curd (approximately 1 cup) inside the dam. Top with plain cake layer, face-up; repeat frosting around perimeter and curd filling. Top with remaining blueberry cake, face-down. Spread a thin layer (crumb coat) of frosting over top and sides of cake. Place cake in refrigerator for about 30 minutes to chill. Remove from refrigerator, and using an offset spatula, spread a final coat of frosting all over cake. Finish with piping details as desired.