In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.

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

Pour the wet ingredients into the dry ingredients and mix gently until combined.

Slowly stir in the hot water or hot coffee. The batter will be thin.

Divide the batter evenly between the cupcake liners, filling each about two-thirds full.

Bake for 16 to 18 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.

Cool the cupcakes in the pan for 5 minutes, then transfer to a wire rack to cool completely.

To make the frosting, beat the softened butter until smooth and creamy.

Gradually add the powdered sugar and beat until combined.

Add the heavy cream, vanilla extract, and salt. Beat until fluffy and smooth.

Pipe or spread the buttercream onto the cooled cupcakes.

Decorate with sprinkles, chocolate curls, or mini chocolate chips if desired.