Chocolate Breakfast Muffins

Ingredients

• 1⁄3 cups Dutch-process cocoa
• 1 cup all-purpose flour
• 3⁄4 cups light brown sugar (packed)
• 1⁄2 teaspoons baking powder
• 1⁄2 teaspoons baking soda
• 1⁄4 teaspoons salt
• 1⁄2 cups chocolate chips
• 1 large egg
• 1⁄3 cups milk
• 1 teaspoon vanilla extract
• 1 teaspoon vinegar
• 4 tablespoons unsalted butter (melted)
• sparkling white sugar (or pearl sugar, for topping, optional)