2/3 cup (2 ounces) Dutch-process cocoa
2 cups (8 1/2 ounces) King Arthur Unbleached All-Purpose Flour
1 1/4 cups (9 3/8 ounces) light brown sugar
1 teaspoon baking powder
3/4 teaspoon espresso powder, optional
1 teaspoon baking soda
3/4 teaspoon salt
1 cup (6 ounces) chocolate chips
2 large eggs
3/4 cup (6 ounces) milk
2 teaspoons vanilla
2 teaspoons vinegar
1/2 cup (4 ounces, 1 stick) butter, melted; or 1/3 cup (2 3/8 ounces) vegetable oil
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 tin; the cups will be heaped with batter, and the muffin will bake into a "mushroom" shape. Sprinkle with pearl sugar, if desired.
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