In a saucepan, whisk the 2 cups of the milk and sugar together. Place the pan over medium heat and bring the liquid up to a simmer. Whisk the egg yolks together. Temper the hot milk into the egg yolks. Whisk the egg mixture into the hot milk mixture. In a small bowl, dissolve the cornstarch in the remaining milk, making a slurry. Whisk the slurry into the hot milk mixture. Bring the liquid up to a boil and reduce to a simmer. Cook the mixture, stirring constantly, until the filling is thick, about 4 to 6 minutes. Fold in the vanilla, and butter. Mix well.

Sponge cake:

Preheat the oven to 350 degrees F. In a small sauce pan, heat the milk and 2 tablespoons butter together. Using an electric mixer fitted with a wire whip, combine the eggs and sugar together. Beat on medium-high speed for about 8 minutes, or until the mixture is pale yellow, thick and has tripled in volume. With the machine running slowly add the heated milk. In a mixing bowl sift the flour, baking powder, and salt together. Fold the flour mixture into the egg mixture and mix thoroughly so that there are no lumps and the mixture is smooth. Fold in the vanilla. Grease 2 each 9-inch round cake pans with 2 tablespoons of butter. Sprinkle each with a tablespoon of sugar. Pour the cake batter evenly into the pans and bake for about 25 minutes, or until the cakes spring back when touched. Cool for about 2 minutes. Using a thin knife, loosen the edges of the cakes, then flip onto a wire rack. When the cakes have cooled completely, slice each cake in half. To assemble, place the bottom layer on a 9-inch round cardboard and place it on a wire rack. Spread 1 1/2 cups of the pastry cream evenly on top of the layer. Top with the second layer of cake. Spread 1 1/2 cups of pastry cream on it. Repeat the same process with the third layer. Top with the fourth layer. If necessary, shave off any uneven pieces of cake with a serrated knife so that it is smooth and even on all sides. Frost the entire cake with the chocolate fondant. Place the cake in the refrigerator and chill completely. Slice the cake into 16 slices.