Preheat oven to 350°F (175°C). Grease and flour three 9-inch round cake pans.

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

Add buttermilk, oil, eggs, and vanilla. Beat with an electric mixer on medium speed for 2 minutes.

Gradually add hot water, mixing until smooth. Batter will be thin.

Pour batter evenly into prepared pans. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Let cool in pans for 10 minutes before inverting onto a wire rack to cool completely.

In a large bowl, beat butter until creamy. Gradually add powdered sugar and cocoa powder, alternating with heavy cream, beating until smooth.

Stir in vanilla extract.

Level the cake layers with a serrated knife if necessary. Place one layer on a serving plate and spread with frosting. Repeat with remaining layers.

Frost the entire cake with the remaining frosting. Decorate as desired (e.g., with fondant decorations, edible glitter, etc.).