Not for the faint of heart!
Recipe from ‘Behind the Scenes with America’s Test Kitchen’
The filling can be made a day ahead and refrigerated, but it will become quite stiff; fold it with a rubber spatula to loosen it before spreading it on the cake. Leftover cake can be stored covered in the refrigerator, with the cut side of the cake covered tightly with plastic wrap, for up to three days.
Lemon Curd Filling
1 cup juice from about 6 lemons
1 teaspoon powdered gelatin
1½ cups sugar
1/8 teaspoon salt
4 large eggs
6 large egg yolks (reserve the whites for cake)
1 stick unsalted butter, cut into ½ inch cubes and frozen
2¼ cups cake flour, plus extra for the pans
1 cup whole milk, room temperature
6 large egg whites, room temperature
2 teaspoons vanilla extract
1¾ cups sugar
4 teaspoons baking powder
1 teaspoon salt
1½ sticks unsalted butter, cut into 12 pieces, softened but still cool
2 large egg whites
1 cup sugar
¼ cup water
1 tablespoon juice from 1 lemon
1 tablespoon corn syrup
1. FOR THE FILLING: Measure 1 tablespoon of the lemon juice into a small bowl; sprinkle the gelatin over the top. Heat the remaining lemon juice, the sugar, and salt in a medium nonreactive saucepan over medium-high heat, stirring occasionally, until the sugar dissolves and the mixture is hot but not boiling. Whisk the eggs and the yolks in a large nonreactive bowl. Whisking constantly, slowly pour in the hot lemon-sugar mixture into the eggs, then return the mixture to the saucepan. Cook over medium-low heat, stirring constantly with a heatproof spatula, until the mixture registers 170 degrees on an instant read thermometer and is thick enough to leave a trail when the spatula is scraped along the pan bottom, 4 to 6 minutes. Immediately remove the pan from the heat and stir in the gelatin mixture until dissolved. Stir in the frozen butter until incorporated. Pour the filling through a fine-mesh strainer into a nonreactive bowl (you should have 3 cups). Cover the surface directly with plastic wrap; refrigerate until firm enough to spread, at least 4 hours.
2. FOR THE CAKE: Adjust an oven rack to the middle position and heat the oven to 350F. Grease and flour two 9 x2 round cake pans and line with parchment paper. In a 2-cup measure or medium bowl, whisk together the milk, egg whites and vanilla.
3. In the bowl of a standing mixer fitted with a paddle attachment, mix the flour, sugar, baking powder, and salt at low speed. With the mixer running at low speed, add the butter one piece at a time; continue beating until the mixture resembles moist crumbs with no visible butter chunks. Add all but ½ cup of the milk mixture to the crumbs and beat at medium speed until the mixture is pale and fluffy, about 1 ½ minutes. With the mixer running at low speed, add the remaining ½ cup milk mixture; increase the speed to medium and beat for 30 seconds more. Stop the mixer and scrape the sides of the bowl. Return the mixer to medium and beat for 20 seconds longer. Divide the batter evenly between the cake pans; using a rubber spatula, spread the batter to the pan walls and smooth the tops.
4. Bake until a toothpick inserted in the center of the cakes comes out clean, 23 to 25 minutes. Loosen the cakes from the sides of the pans with a small knife, cool in the pan for 10 minutes, then invert onto a greased wire rack; peel off the parchment. Invert the cakes again; cool completely on the rack, about 1 1/2 hours.
5. TO ASSEMBLE: Use a serrated knife to cut each cake into 2 even layers. Place the bottom layer of one cake on a cardboard round or cake plate. Using an icing spatula spread 1 cup of the lemon filling evenly on the cake leaving a ½-inch border around the edge, using a cardboard round, gently replace the top layer. Spread 1 cup of filling on top. Using the cardboard round, gently slide the bottom half of the second cake into place. Spread the remaining cup of filling on top. Using the cardboard round, replace the top layer of the second cake. Smooth out any filling that has leaked from the sides of the cake; cover with plastic wrap and refrigerate while making the icing.
6. FOR THE ICING: Combine all the ingredients in a large heatproof bowl and set over a medium saucepan filled with 1 inch of barely simmering water (do not let the bowl touch the water). Cook, stirring constantly, until the mixture registers 160 degrees on an instant-read thermometer, 5 to 10 minutes. Remove the bowl from the heat and transfer the mixture to a standing fitted with a whisk attachment. Beat on medium speed until soft peaks from, about 5 minutes. Increase the speed to medium-high and continue to beat until the mixture has cooled to room temperature and stiff peaks form, 5 minutes longer. Ice the cake and serve.