Preheat the oven to 350°F. Wash and dry four food-safe, oven-safe 15-ounce metal cans, remove all labels, grease the interiors thoroughly, and line the bottoms with small parchment circles.

Whisk flour, sugar, cocoa, baking soda, baking powder, and salt in a bowl. Add the egg, buttermilk, oil, and vanilla, then whisk until smooth before stirring in the hot coffee.

Divide the batter among the prepared cans, filling each one no more than halfway. Place the cans upright on a baking sheet and bake for 23 to 27 minutes, until a toothpick comes out with a few moist crumbs.

Cool the cakes in the cans for 15 minutes, then loosen the edges and carefully remove them. Cool completely before frosting.

Beat the butter until creamy, then gradually mix in powdered sugar and cocoa. Add cream, vanilla, and salt, beat until fluffy, and spread or pipe the frosting over the cooled cakes.

Finish with chocolate curls or sprinkles. Serve immediately, or chill the cakes briefly for cleaner frosting and easier transport.