1. Preheat the oven to 350°F. Grease two 8-inch round cake pans, line the bottoms with parchment paper, and lightly grease the parchment.

2. In a large mixing bowl, whisk together the flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt.

3. In a separate bowl, mix the eggs, sour cream, milk, vegetable oil, and vanilla extract until smooth.

4. Add the wet mixture to the dry ingredients and stir until mostly combined.

5. Slowly pour in the hot coffee or hot water and mix until the batter is smooth and evenly blended.

6. Divide the batter evenly between the prepared pans.

7. Bake for 28 to 34 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

8. Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

9. For the frosting, beat the softened butter until creamy.

10. Mix in the melted chocolate, then gradually add the powdered sugar and heavy cream until the frosting is smooth and fluffy.

11. Place one cake layer on a serving plate and spread the chocolate ganache or thick chocolate pudding filling over the top.

12. Add the second cake layer and frost the top and sides of the cake.

13. Chill for 20 to 30 minutes before slicing for cleaner layers, then serve.