Make the Cake Batter

1. Preheat the Oven: Start by preheating your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, or line them with parchment paper for easy removal later.
2. Mix the Dry Ingredients: In a large bowl, sift together the flour, black cocoa powder, baking soda, baking powder, and salt. Set aside.
3. Mix the Wet Ingredients: In a separate bowl, whisk together the granulated sugar, brown sugar, eggs, buttermilk, vegetable oil, and vanilla extract. Mix until smooth and fully combined.
4. Combine Wet and Dry Ingredients: Gradually add the wet ingredients to the dry ingredients, stirring until just combined. Don’t overmix.
5. Add Boiling Water: Slowly pour the boiling water into the batter and mix gently. The batter will be thin, but that’s perfectly fine—it helps to create a moist, dense cake.
6. Bake: Divide the batter evenly between the prepared cake pans. Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean. Keep a close eye on it to avoid overbaking, as that can dry out the cake.
7. Cool: Let the cakes cool in the pans for 10 minutes, then turn them out onto a wire rack to cool completely.

Make the Frosting

1. Beat the Butter: In a large mixing bowl, beat the softened butter until smooth and creamy using an electric mixer.
2. Add the Dry Ingredients: Gradually add the powdered sugar and black cocoa powder, beating on low speed until combined.
3. Add Vanilla and Cream: Add the vanilla extract and 3 tablespoons of heavy cream. Beat until smooth and fluffy. If the frosting is too thick, add a bit more cream until you reach the desired consistency.

Assemble the Cake

1. Frost the First Layer: Once the cakes are completely cool, place one layer on a serving platter. Spread a generous layer of frosting over the top.
2. Add the Second Layer: Place the second cake layer on top and frost the top and sides of the entire cake. Smooth the frosting with a spatula or offset knife for a professional finish.
3. Decorate: If you like, you can add some chocolate shavings or sprinkles on top for a little extra flair.