Preheat the oven to 350°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, espresso powder, baking soda, salt, and chocolate chips. 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., Scoop the batter into the prepared muffin pan; the cups will be heaped with batter, and the muffin will bake into a "mushroom" shape. Sprinkle with pearl sugar., 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.