Preheat the oven to 350 degrees. Grease three 6" cake pans and line the bottoms with parchment paper rounds. Set aside.

In a medium bowl, whisk together the flour, black cocoa powder, baking powder, baking soda, and salt.

In a large bowl, cream together the unsalted butter and granulated white sugar for 1-2 minutes with an electric mixer on high speed until fluffy.

Add the eggs and vanilla to the butter mixture and mix for 1-2 minutes on medium speed until pale in color.

Gradually add the dry ingredients and buttermilk to the butter/sugar/egg mixture on medium-low speed until combined.

Mix in the hot coffee and a few drops of black food coloring on medium-low speed.

Divide the batter evenly between the three prepared cake pans.

Bake the cakes for 38-44 minutes, or until a toothpick comes out clean from the centers with a few moist crumbs.

Let the cakes cool in the pans for five minutes, then transfer them to a cooling rack to cool completely.

While the cakes are cooling, prepare the frosting by mixing the softened unsalted butter and salt on high speed until pale and fluffy, about 5-10 minutes.

Sift in the black cocoa powder and mix on medium-low speed until combined.

Sift in the powdered sugar a cup at a time until fully combined. The frosting will be thick.

Add heavy cream, a tablespoon at a time, mixing on high speed until light and fluffy, about 1-2 minutes.

Mix in a few drops of black food coloring, if desired.

Once the cakes are cooled, fill each layer and frost the outside with the black cocoa buttercream. Add sprinkles to the top and outside of the cake.