Preheat the oven to 350ºF and prepare three 4-inch or one 6-inch cake pan by spraying the sides with cooking spray and fitting the bottom(s) with a wax paper or parchment paper cake circle.

Place all of the dry ingredients into a medium bowl and whisk them together. Add the egg, vegetable oil, vanilla, and buttermilk and whisk until fully combined. Add the hot coffee (or hot water) in a slow stream and whisk until fully combined and smooth. The batter will be very thin.

Pour the batter into the prepared cake pan(s). If using 4-inch cake pans, bake for 18-22 minutes. If using a 6-inch cake pan bake for 24-28 minutes. The cake is done when it springs back to the touch and a toothpick inserted comes out clean. Cool completely before assembling and decorating.

In a stand mixer fitted with a paddle attachment, whip the butter on medium-high speed until creamy and light in color, 4-5 minutes.

Scrape down the bowl and paddle, then add the powdered sugar, cocoa powder, milk, salt, and vanilla extract. Cover the mixer with a clean kitchen towel (to prevent ingredients flying out of the bowl), then turn the mixer to low speed and mix until all ingredients start to come together.

Remove the kitchen towel, scrape down the bowl and paddle, then continue mixing on low speed until uniform and smooth, 2-3 minutes.

Once the chocolate cake is completely cooled, frost and decorate with the chocolate buttercream. To create the mini layer cake pictured, level the cake layers to your desired height. Add a swipe of chocolate buttercream onto a cardboard cake circle and place the first cake layer on top. Fill and stack the cake with chocolate buttercream, then crumb coat the cake with chocolate buttercream. Place the crumb coated cake in the refrigerator for at least 30 minutes to let the buttercream set firm.

Use the remaining frosting to create a smooth buttercream finish and pipe a star border on top using Wilton Tip 4B. Decorate with sprinkles (I used Sprinkle Pop's "Hey Sugar" mix).