These moist and dense cake-style doughnuts with their deep-dark chocolate flavor will start any chocolate lover's morning off right. Their rich, fudge-like texture and pockets of melted chocolate also make them suitable for a decadent bite-sized snack or dessert; but be warned that you may not be able to stop at just one!
Preheat the oven to 350°F. Lightly grease the top and bottom halves of a doughnut hole pan.
Whisk together the cocoa, flour, sugar, baking powder, espresso powder, baking soda, salt, and chocolate chips. Set aside.
Whisk together the egg, milk, vanilla, and vinegar. You may notice some curdling of the milk; that's OK.
Add the wet ingredients, along with the melted butter, to the dry ingredients, stirring to blend.
Scoop a tablespoon of batter into the cups in the bottom half of the doughnut hole pan (the side without the vent hole) — a level tablespoon cookie scoop works well here.
Place the top half on the pan, and lock it in place with the clips.
Bake the doughnut holes for 8 to 9 minutes, or until a cake tester inserted into the center of one comes out clean.
Remove the pan from the oven, and allow it to cool slightly before removing the clips and the top half.
Loosen the edges of the holes, if necessary, and gently turn them out onto a rack.
Immediately shake the holes in the coating, and set aside to cool.
Grease the pan again, and repeat the process with the remaining batter.
Store doughnuts, tightly wrapped, at room temperature for several days; freeze for longer storage.
If you don't have a doughnut hole pan, a mini muffin pan also works well. Portion a level tablespoon cookie scoop (about 4 teaspoons) into the greased mini muffin cups, and bake in a preheated 425°F oven for 7 minutes, or until a cake tester or toothpick inserted into the center of a muffin comes out clean.
