Preheat oven to 350°F. Grease and flour two 9-inch round cake pans. In a large bowl, combine flour, baking powder, baking soda, and salt. Set aside. In another bowl, cream butter and sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Stir in vanilla. Alternately add the dry ingredients and milk to the butter mixture, beginning and ending with dry ingredients. Mix until just combined.

Divide batter evenly between the prepared pans. Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Allow cakes to cool in pans for 10 minutes, then turn out onto wire racks to cool completely.

In a saucepan, whisk together egg yolks, sugar, and cornstarch until smooth. Gradually whisk in milk. Cook over medium heat, stirring constantly, until mixture thickens and comes to a boil. Remove from heat and stir in vanilla. Transfer to a bowl, cover with plastic wrap touching the surface to prevent a skin from forming, and chill until set.

Place chopped chocolate in a heatproof bowl. Heat cream in a saucepan until it just starts to simmer. Pour over chocolate and let sit for 5 minutes. Stir until smooth.

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

Chill the assembled cake for at least 1 hour before serving to allow the ganache to set. Slice and serve with coffee or tea.

Store leftover cake covered in the refrigerator for up to 3 days. Allow to come to room temperature before serving for best flavor.