Heat the oven to 350°F (175°C) and line a 12-cup muffin pan with paper liners. Whisk the flour, cocoa powder, sugar, baking soda, baking powder, and salt in a bowl.

In a separate bowl, whisk the room-temperature egg, buttermilk, oil, vanilla, and coffee liqueur. Add the wet mixture to the dry ingredients and stir just until combined, then stir in the hot coffee; the batter will be fairly thin.

Divide the batter among the liners, filling each about two-thirds full. Bake for 18-22 minutes, until the tops spring back lightly and a toothpick comes out with a few moist crumbs.

Cool the cupcakes in the pan for about 5 minutes, then transfer them to a rack and cool completely. For easier decorating, chill the cooled cupcakes for 15-20 minutes.

Beat the softened butter until pale and fluffy, about 2-3 minutes. Gradually beat in the sifted confectioners' sugar, then add the Irish cream, vanilla, and salt; beat until smooth and add heavy cream or milk only if needed.

For the optional ganache, heat the heavy cream until steaming but not boiling and pour it over the chopped chocolate. Let it stand for 2 minutes, stir until smooth, mix in the coffee liqueur, and cool until thickened but still spreadable.

Pipe or spread the ganache and Irish cream frosting over the completely cooled cupcakes. Finish with chocolate curls, chocolate-covered coffee beans, or a light drizzle of coffee liqueur or chocolate sauce.