Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans.

In a mixing bowl, beat together the unsalted butter and granulated sugar until light and fluffy, about 3 minutes.

Add the large eggs one at a time, mixing well after each addition, and stir in the pure vanilla extract.

Sift in the all-purpose flour and cornstarch, mixing until just combined.

Divide the batter evenly between the prepared pans and bake for 25-30 minutes, or until a toothpick comes out clean.

For the custard, whisk together whole milk, sugar, cornstarch, egg yolks, and vanilla in a saucepan over medium heat, stirring continuously until thickened.

For the glaze, melt the bittersweet chocolate with the heavy cream in a small saucepan over low heat until smooth.