For a Black Velvet Cake, preheat the oven to 350 degrees. In a large bowl, combine chocolate cake mix, chocolate pudding mix, and black cocoa.
Add eggs, vegetable oil, milk, buttermilk, and brewed coffee, and mix with an electric mixer until you have a smooth batter.
Grease a bundt pan, pour in the batter, and bake for 35-40 minutes until a toothpick comes out with few crumbs (no raw batter).
Allow it to cool in the pan for 15 minutes, then invert it onto a cake plate and let it cool completely for about 45 minutes.
For Black Cocoa Frosting, melt butter, black cocoa, and milk in a saucepan over medium heat until smooth.
Remove from heat, add powdered sugar, and whisk until smooth.
Allow the frosting to thicken slightly for 2 minutes.
Then, pour it over the cooled cake and add sprinkles on top.