Special equipment: a 12-cup standard muffin tin or two 24-cup mini-muffin tins, cupcake liners
Preheat the oven to 350 degrees F and position a rack in the middle of the oven. Line one 12-cup standard muffin tin or two 24-cup mini-muffin tins with cupcake liners.
Combine the butter, cocoa, coffee and 1 cup water in a microwave-safe bowl. Cover with plastic wrap and microwave on high until the butter melts, about 1 minute. Whisk to combine.
Meanwhile, whisk the flour, sugar, baking powder, baking soda and salt in a large bowl. In a small bowl, beat the sour cream, vanilla and eggs. Whisk the hot cocoa mixture into the dry ingredients. Then stir in the sour cream mixture just to combine; don't overmix.
Divide the batter evenly in the prepared muffin tin, filling each cup about three-quarters full. Bake until a toothpick inserted into the center of the cupcakes comes out clean, 18 to 20 minutes for standard cupcakes or 16 to 18 minutes for minis.
Cool the cupcakes in the tin on a rack for 10 minutes, then remove from the tin and cool on the rack completely. Frost and decorate as desired.