In a large mixing bowl, whisk together the cocoa powder, flour, sugar, baking powder, espresso powder, baking soda, salt, and chocolate chips. Set aside.
In a medium-sized mixing bowl, whisk together the eggs, milk, vanilla, and vinegar.
Add the wet ingredients to the dry ingredients, stirring until just combined. Be sure to not over-stir, as this will cause a tough muffin.
Cover bowl and set in refrigerator for at least an hour or overnight.
Preheat the oven to 425°F. Line a standard muffin pan with paper or silicone muffin cups, and grease the cups.
Using a large cookie scoop, scoop the batter into the prepared muffin pan. Sprinkle with coarse sugar. Bake in preheated oven at 425 for 6-10 minutes then reduce oven heat to 350 degrees (without opening the oven door). Bake an additional 8-10 minutes or until toothpick inserted in middle comes out clean.
Remove muffins from oven and place on cooling rack for 5 minutes before removing from pan and setting on rack to cool completely.