Preheat the oven to 350 degrees F. Grease and line 2 (8-inch) cake pans.
For the cake: Whisk together the eggs, egg whites, milk, and vanilla and set aside.
Place the sugar, flour, baking powder, and salt in the bowl of a stand mixer fitted with a paddle attachment. Add in the softened butter and half of the egg mixture to the mixer and beat on medium speed until batter is very smooth, about 4 minutes.
Add the remaining egg mixture in 3 additions, scraping down the bowl after each addition, and beating for 2 minutes afterwards.
Portion out the batter evenly into the prepared pans. Bake until the cake springs back when lightly touched in center, 45 to 50 minutes. Cool the cakes in the pan for 10 minutes, and then turn out to cool completely. Can be made a day ahead.
For the ganache: Place the ingredients into a heatproof bowl and place over a saucepan of barely simmering water. Whisk until melted and smooth.
Place the cooled cake on a cake round. Trim the top of the cake and hollow out the middle to hold the Pastry Cream. Cut the trimmed cake into big chunks and set aside. Place the lightened Pastry Cream into the cake, creating a small mound in the middle. Pile the cake pieces on top of the Pastry Cream. Pour warm ganache over the entire surface until covered. Let the cake rest for 2 to 3 hours before serving.
Place most of the milk, half of the sugar, and vanilla into a saucepan and bring to a low simmer. Meanwhile, whisk together the rest of the milk, eggs, yolks, the remaining sugar, and cornstarch. Temper the egg mixture with 1/3 of the hot milk, whisk well, and pour back into the saucepan. Over medium heat, continue to cook and whisk until the custard is thick and starts to bubble. Continue whisking and cooking for 10 more seconds. Strain the pastry cream through a sieve, pushing through with a spatula, into a clean mixing bowl. Immediately cover the entire custard surface with plastic wrap and cool before using.
When the pastry cream is cool, remove the plastic wrap, insert the beater attachment into the stand mixer, and beat the pastry cream on medium speed until smooth. Remove the cream to a mixing bowl. Whisk the heavy cream to stiff peaks and fold into the pastry cream in 2 additions.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Eunice Feller, Bread and Chocolate Bakery Cafe, Newton Highlands, MA