Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners. Whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt. In a separate bowl, combine egg, buttermilk, oil, and vanilla.
Gradually mix the dry ingredients into the wet ingredients until just combined. Slowly add hot coffee and stir until smooth. Fill cupcake liners ⅔ full and bake for 18-20 minutes. Cool in the pan for 5 minutes, then transfer to a wire rack.
Beat butter until creamy. Gradually mix in powdered sugar, cocoa powder, cream, and vanilla. Beat until light and fluffy.
Frost cooled cupcakes and serve after 10 minutes to let the flavors meld.