Preheat your oven to 350°F (175°C). Grease and line two 9-inch round cake pans.
In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
In another bowl, beat the eggs, buttermilk, warm water, oil, and vanilla until smooth. Add black food coloring if desired.
Slowly add the wet ingredients to the dry mixture. Beat until smooth and well combined.
Divide the batter evenly into the prepared pans. Bake for 30-35 minutes or until a toothpick comes out clean.
Allow the cakes to cool completely on a wire rack.
Beat the butter until creamy. Gradually add powdered sugar, cocoa powder, heavy cream, vanilla, and a pinch of salt. Beat until fluffy.
Layer the cakes with frosting. Spread evenly on top and sides.
Slice, serve, and enjoy your stunning Black Velvet Cake!