1. Whisk sugar, cornstarch, egg yolks, and egg in a bowl until smooth.
2. Simmer milk, sugar, and vanilla in a saucepan. Temper egg mixture with hot milk.
3. Cook mixture until thickened. Strain and whisk in butter. Chill pastry cream.
4. Preheat oven to 350ºF. Prepare two 8-inch cake pans.
5. Mix flour, sugar, baking powder, baking soda, salt, and butter until crumbly.
6. Add eggs, yolks, milk, oil, and vanilla. Mix until smooth.
7. Divide batter into pans. Bake for 30-35 minutes. Cool cakes on a wire rack.
8. Melt chocolate with cream for ganache. Cool slightly.
9. Assemble cake: Layer cake, pastry cream, cake, and top with ganache.