Lemon Layer Cake

Lemon Layer Cake

Filling:
1 cup juice from about 6 lemons
1 teaspoon powdered gelatin
1 1/2 cups sugar
1/8 teaspoon salt
4 large eggs, plus 6 large egg yolks (reserve the egg whites for the cake)
8 tablespoons unsalted butter

Cake:
2 1/4 cup cake four
1 cups whole milk, room temperature
6 large egg whites, room temperature
2 teaspoons vanilla
1 3/4 cup sugar
4 teaspoons baking powder
12 tablespoons unsalted butter, cut into 12 pieces, softened but still cool

Filling:  Measure 1 tablespoon of the lemon juice into a small bow; sprinkle the gelatin over the top.  Heat the remaining lemon juice, the sugar, and salt in a medium saucepan over medium-high heat, stirring occasionally, until the sugar dissolves and the mixture is hot but not boiling.  Whisk the eggs and yolks in a large bowl.  Whisking constantly, slowly pour 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 spatual is scraped along the pan bottom, 4-6 minutes.  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 bowl ( you should have 3 cups).  Lay a sheet of plastic wrap directly on the surface and refrigerate until firm enough to spread, at least 4 hours.

Cake:  Adjust an oven rack to the middle position and heat the oven to 350 degrees.  Grease and flour two 9-inch cake pans and line with parchment.  In a 2-cup liquid measure, whisk together the egg whites, and vanilla.

In a stand mixer fitted with the paddle attachment, mix the flour, sugar, baking powder and salt at low speed until combine, about 30 seconds.  With the mixer running at low speed, add the butter 1 piece at a time; continue beating until the mixture resembles moist crumbs with no visible butter chunks.  Add all but 1/2 cup of the milk mixture to the crumbs and beat at medium speed until the mixture is pale and fluffy, about 1 1/2 minutes.  With the mixer running at low speed, and the remaining 1/2 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 speed and beat for 20 seconds longer.  Divide the batter evenly between the pans, lightly tapping the pans against the countertop 2 or 3 times to settle the batter.

Bake until a toothpick inserted in the center of the cakes comes out clean, 23 to 25 minutes, rotating the pans halfway through the baking time.  Cool the pans on a wire rack for 10 minutes.  Run a knife around the edges of the cakes, then flip them out onto a wire rack.  Peel off the parchment, flip the cakes, and cool completely before frosting, about 2 hours.

Assemble:  Line the edges of a cake platter with strips of parchment.  Use a serrated knife to cut each cake horizontally into 2 even layers.  Place the bottom layer of 1 cake on the platter.  Using a spatula, spread 1 cup of the filling evenly on the cake, leaving a 1/2 inch border around the edge.  Carefully place the upper cake layer on top of the filling.  Spread 1 cup of the filling on top; repeat, using the remaining filling and cake layers.  Cover with plastic wrap and refrigerate while making the icing.

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, 5-10 minutes.  Remove the bowl from the heat and transfer the mixture to a stand mixer fitted with the whisk attachment.  Beat on medium speed until soft peaks form, 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.  Using a spatula, spread the frosting evenly over the top and sides of the cake.  Remove the parchment strips from the platter and serve.