Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a large bowl, mix together the flour, sugar, and baking powder. Add butter, eggs, vanilla extract, and milk. Beat until smooth and combined.

Pour the batter evenly into the prepared pans. Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Allow the cakes to cool.

While the cake is cooling, prepare the custard. Heat the milk and heavy cream in a saucepan over medium heat. In a separate bowl, whisk the egg yolks, sugar, and cornstarch. Slowly add the warm milk mixture to the egg mixture, whisking constantly.

Return the custard to the stove and cook over medium heat until it thickens. Remove from heat and stir in the vanilla extract. Let it cool completely.

For the glaze, melt the dark chocolate, heavy cream, and butter together in a heatproof bowl over simmering water. Stir until smooth.

Once the cakes have cooled, place one layer on a serving plate and spread the cooled custard filling over it. Top with the second cake layer. Pour the chocolate glaze over the top, allowing it to drip down the sides.

Refrigerate the cake for a few hours to allow the glaze to set before serving.