3/4cup (1½ sticks)unsalted butter,at room temperature
LEMON TRUFFLE FILLING:
3/4cupgranulated white sugar
1/3cupfreshly squeezed lemon juice
1teaspoongrated lemon zest
WHIPPED CREAM FROSTING:
Preheat the oven to 350 F. Grease and flour two 9-inch round cake pans.
PREPARE THE CAKE:
In a large bowl with an electric mixer, combine the flour, sugar, milk, butter, baking powder and salt. Blend until the butter is evenly mixed. Add the vanilla. Beat at low speed for 1 minute, then increase the speed to high and beat for 2 minutes, scraping the bowl constantly, beating until the mixture is light and fluffy.
In another bowl, beat egg whites until stiff. Gently but thoroughly fold the whites into the cake batter. Pour evenly into the prepared pans.
Bake 35 to 40 minutes, or until the cakes are no longer jiggly in the center. Remove from the oven and cool on a wire rack. Split the cooled layers horizontally to make 4 layers in all. (I use a serrated knife… and gently saw around the center to cut even layers).
PREPARE THE FILLING:
In a medium saucepan, combine sugar, cornstarch, and salt. Whisk in water until smooth. Cook over medium-high heat until the mixture comes to a boil, stirring constantly. Reduce the heat to low and cook 2 minutes longer, stirring constantly. Remove from the heat. In a small bowl, beat the egg yolks with a fork. Slowly stir about 1/4 cup of the hot mixture into the egg yolks, whisking quickly while adding the hot liquid. (You don’t want to create scrambled eggs, so be sure to add the hot mixture slowly, whisking it into the eggs as quickly as possible). Continue adding 1/4 cups of the hot mixture to the eggs until you have warmed up the egg mixture. Whisk the egg yolk mixture back to the mixture in the saucepan. Cook over low heat until the mixture boils, whisking constantly, then continue to cook for 2 minutes, stirring.
Remove from the heat and stir in the lemon juice, butter and zest. Transfer half of the hot filling to a small bowl and set aside to cool. Add the white chocolate to the hot filling in the saucepan and stir until the chocolate is melted; set aside. In a small bowl, beat the cream cheese until fluffy. Beat in the white chocolate mixture until light and smooth. At this point, you should have a small bowl of lemon curd and a bowl of white chocolate truffle filling.
Spread one third of the white chocolate filling over one layer of the cake. Top with a second cake layer, then spread with one third more of the filling; top with a third cake layer and spread the remaining filling over. Top with the last cake layer and spread reserved lemon curd. Refrigerate, uncovered, until the filling is set (2 to 3 hours).
MAKE THE WHIPPED CREAM:
In another bowl, whip the cream and sweeten it with the powdered sugar. Frost the sides of the cake with half of the whipped cream frosting. Put the remaining frosting into a pastry bag and pipe decoratively around the top and bottom edge of the cake. Refrigerate up to 8 hours before serving.