The flavors of chocolate and espresso combine to make this an ideal after-dinner treat from the dessert buffet.
Preheat oven to 350 degrees. Generously butter four 12-count mini muffin tins, and set aside. Sift flour, baking soda, baking powder, and salt into a small bowl; set aside. Warm milk slightly in a small saucepan; stir in cocoa and espresso powders until both are dissolved. Let cool completely.
Add butter and granulated sugar to the bowl of an electric mixer fitted with the paddle attachment; cream on medium speed until pale and fluffy, about 2 minutes. Add eggs one at a time, beating after each addition until fully combined; mix well, about 2 minutes.
On low speed, add flour and milk mixtures in alternating batches, beginning and ending with flour mixture. Mix until just combined. Place about 1 tablespoon batter in each muffin cup.
Bake cakes until a cake tester inserted into center comes out clean, about 15 minutes. Let cool in pans on wire racks for 10 minutes. Run a knife around edges of cakes; invert onto racks. Let cool.
Trim tops of cakes with a serrated knife so that they will lie flat when turned upside down. Just before serving, dust bottoms of cakes with confectioners' sugar (use a stencil if desired).