Preheat oven to 350. Line your muffin pan with liners or spray with bakers joy.
In a mixing bowl, whisk together the cocoa, flour, sugar, baking powder, espresso powder, baking soda, and salt. In a large measuring cup or medium-sized mixing bowl, whisk together the eggs, milk, vanilla, and vinegar.
Add the wet ingredients, along with the melted butter, to the dry ingredients, stirring to blend; there's no need to beat these muffins, just make sure everything is well-combined. Fold in the chocolate chips.
Scoop the batter into the prepared muffin pan. This recipe makes 12 full sized muffins and the cups will be full and bake up with a mushroomed top. It takes about 24 mini muffins and cups will also be fairly full.
Bake the muffins for 15-20 minutes for full size, 10 for mini, or until a cake tester inserted in the center of a muffin comes out clean. My oven runs hot, so use the cake tester and adjust time for your oven. These can burn easily on the edges so you need to watch them closely and cook until just done. They smell like brownies when they are done!
Remove the pan to a cooling rack, wait 5 minutes and then turn them out to cool further out of the pan. You will not regret making these, I promise.