Preheat the oven to 350 degrees Fahrenheit (180 degrees Celsius). Then line a metal mini cupcake pan with paper liners and set aside.

In a separate bowl, whisk the egg, oil, buttermilk, and vanilla extract together until well combined.

Pour the wet ingredients into the bowl of dry ingredients. Gently fold with a rubber spatula. There will be lumps, but keep folding until the mixture is mostly smooth.

Pour half of the hot coffee into the batter, gently folding it in with your spatula. Repeat with the remaining coffee until smooth and thin

Carefully spoon the cupcake batter in the mini cupcake liners. Fill each liner only ½-2/3 full so they don't overflow in the oven.

Bake for 11-14 minutes or until a toothpick inserted in the center of a cupcake comes out clean with moist crumbs (no wet batter).

Cream softened butter in the bowl of an electric mixer or stand mixer with the paddle attachment. Beat on medium-high speed for 2-3 minutes until smooth.

Add the cocoa powder and salt, mixing on low until creamy. Scrape down the sides of the bowl.

Add the powdered sugar one cup at a time on low speed. The mixture will be crumbly like sand.

Increase to medium speed and stream in heavy cream. Increase the speed to medium-high, whipping the cream until the frosting reaches a pipeable consistency.

Transfer the frosting to a piping bag fitted with a piping tip and frost the cupcakes. Decorate with sprinkles, chocolate shavings, or mini chocolate chips and enjoy!