Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.

Whisk together the flour, cocoa powder, baking soda, baking powder, and salt. Set aside 1 tablespoon of this mixture in a small bowl.

In a separate bowl, whisk the melted butter, granulated sugar, and brown sugar until glossy.

Whisk in the eggs one at a time, then the vanilla and buttermilk.

Fold the dry ingredients into the wet mixture in two additions, just until no flour streaks remain. Stir in the hot coffee until smooth.

Toss the mini chocolate chips in the reserved flour mixture, then fold into the batter.

Divide the batter evenly among the liners, filling each two-thirds full.

Bake for 18–20 minutes, until a toothpick comes out with a few moist crumbs. Cool in the pan 5 minutes, then transfer to a wire rack to cool completely.

Beat the softened butter until pale and fluffy, about 2 minutes. Add the powdered sugar, cocoa powder, heavy cream, vanilla, and salt, and beat until smooth.

Pipe or spread the buttercream onto the fully cooled cupcakes and top with extra mini chocolate chips.