Preheat oven to 350°F (175°C). Grease and flour two 8-inch round cake pans.

1. In a bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
2. In another large bowl, cream butter and sugar until light and fluffy. Add eggs one at a time, then mix in milk and vanilla.
3. Gradually add dry ingredients to wet ingredients, mixing until a smooth batter forms.
4. Divide batter evenly into the prepared pans and bake for 28–32 minutes or until a toothpick comes out clean. Cool completely.
5. To make pudding, combine sugar, cornstarch, milk, and water in a saucepan. Whisk and cook over medium heat until thickened.
6. Add chopped chocolate and stir until melted and smooth. Let pudding cool to room temperature.
7. Slice each cake layer in half horizontally to create four thin layers. Set one aside to crumble for topping.
8. Layer remaining three cake layers with chocolate pudding between each.
9. Cover cake with pudding and gently press reserved cake crumbs over the top and sides.
10. Chill cake for at least 2 hours before serving.