For the filling
6 large egg yolks (save 4 of the whites for the cake), at room temperature
1 ½ tablespoons grated lemon zest
½ cup freshly squeezed lemon juice
¼ teaspoon lemon extract
1 ½ cups granulated sugar
½ cup (1 stick) unsalted butter, cut into small pieces
For the cake
2 ¼ cups cake flour
1 tablespoon baking powder
½ teaspoon salt
1 cup buttermilk
Juice and grated zest of 1 lemon
4 large egg whites, at room temperature
1 ½ cups granulated sugar
½ cup (1 stick) unsalted butter, at room temperature
½ teaspoon lemon extract
For the frosting
1 cup unsalted butter, at room temperature
4 cups confectioners' sugar
Juice and grated zest of 1 lemon
To make the filling, whisk the egg yolks, lemon zest, lemon juice, lemon extract, and sugar together in a medium saucepan. Set the pot over medium heat and cook, stirring constantly, until the mixture is bubbling and thick, about 20 minutes. Remove the pot from the heat and immediately stir in the butter, one piece at a time, until it melts and is incorporated. Refrigerate overnight.
To make the cake, preheat the oven to 350º Fahrenheit and grease two 9-inch round cake pans with baking spray, or use a combination of cooking spray and flour. Line the bottom of the pans with similarly greased parchment paper.
In a medium bowl, whisk together the flour, baking powder and salt.
In a separate bowl, whisk together the milk, lemon juice and egg whites.
In a large mixing bowl with an electric mixer or in a stand mixer, combine the sugar and lemon zest. Add the butter. With the mixer set on medium speed, cream them together until light and fluffy, about 3 minutes. Add the lemon extract along with some of the flour mixture and beat to combine. Add half of the milk mixture and beat to incorporate, then add more of the flour mixture and beat in. Beat in the remaining milk mixture and then the remaining flour mixture. Beat for 2 minutes more.
Transfer the batter to the prepared cake pans, dividing equally.
Bake for 30 to 35 minutes, until the top of the cake is dry and springy.
Remove the cakes from the oven and let them cool in their pans on wire racks for about 5 minutes. Run a knife around the edge of the cakes to loosen them from the pans and then carefully invert them onto the racks. Let cool completely.
To make the frosting, in a large mixing bowl with an electric mixer or in in the bowl of a stand mixer, cream the butter until fluffy and light, about 2 minutes. Add the confectioners' sugar, lemon juice and lemon zest and beat on medium until light and fluffy, about 3 minutes more.
To assemble the cake, place one of the cake layers on a cake plate and top with the filling, spreading it into an even layer.
Place the second cake layer on top of the first and then spread the frosting all over the top and sides of the cake.
