Best Chocolate Muffins

Preheat the oven to 400°F. Line a standard muffin pan with paper or silicone muffin cups, and grease the cups.
In a large mixing bowl, whisk together the cocoa, flour, sugar, baking powder, baking soda, espresso powder, salt, and chocolate chips. (I put the chocolate chips in last after the rest of the batter was finished). Set aside.
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. (I still had some little bumps in mine. I then stirred in the chocolate chips).
Scoop the batter into the prepared muffin tin; the cups will be heaped with batter, and the muffin will bake into a "mushroom" shape.
Bake the muffins for 20 to 25 minutes, or until a cake tester inserted in the center of a muffin comes out clean. Remove the muffins from the oven, and after 5 minutes remove them from the pan, allowing them to cool for about 15 minutes on a rack before peeling off the muffin papers or silicone cups.