Heat your oven to 350°F (175°C) and line a 12-cup muffin pan with paper liners.
In a big bowl, whisk together the sugar, flour, cocoa powder, baking soda, baking powder, salt, and instant coffee powder until evenly blended.
In another bowl, beat the egg, and then add the oil, buttermilk, vanilla, and hot water. The hot water helps unlock the rich flavors of the cocoa and coffee while smoothing out the batter.
Slowly combine the wet ingredients into the dry mixture, stirring gently until just combined. Mix only until smooth to keep the cupcakes soft and tender.
Spoon the batter into the liners, filling each one about two-thirds full. Bake for 18 to 20 minutes. Check doneness by inserting a toothpick into the center it should come out clean. Allow the cupcakes to rest in the pan for 5 minutes, then transfer to a wire rack to cool completely.
For the frosting, beat softened butter until light and creamy. Gradually add sifted cocoa powder and powdered sugar, alternating with melted chocolate. Stir in heavy cream one tablespoon at a time until the frosting is fluffy and spreadable.
Once cooled, frost the cupcakes with the chocolate buttercream. Enjoy your delicious treats!