Preheat your oven to 350°F. Grease and flour two 9-inch round cake pans. In a large mixing bowl, cream the butter and sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Stir in vanilla. In another bowl, whisk together flour, baking powder, baking soda, and salt. Gradually add to the creamed mixture alternately with milk, beating well after each addition. Divide the batter between the prepared pans.

Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pans for 10 minutes before transferring them to wire racks to cool completely.

For the custard, heat milk in a saucepan over medium heat until hot but not boiling. In a bowl, whisk sugar, cornstarch, and egg yolks until smooth. Gradually whisk the hot milk into the egg mixture. Return to the saucepan and cook over medium heat, stirring constantly, until thickened. Stir in vanilla, then cool completely.

To prepare the ganache, heat cream in a small saucepan until it begins to simmer. Remove from heat and pour over chopped chocolate. Let sit for 5 minutes, then stir until smooth. Let it cool slightly before using.

Place one cake layer on a serving plate. Spread the custard evenly over the top. Place the second cake layer on top. Pour the ganache over the cake, spreading it to the edges and allowing it to drip down the sides.

Chill the assembled cake for at least an hour before serving. Slice and enjoy this decadent dessert at room temperature for the best flavor.

Store the cake in an airtight container in the refrigerator for up to 3 days. For longer storage, you may freeze it for up to a month.