Preheat oven to 425°F and line two 12-count muffin tins with 18 paper liners. Set aside.
In a large bowl, whisk the granulated sugar, melted butter, and vegetable oil together.
1 cup granulated sugar, 6 tablespoons unsalted butter, ¼ cup vegetable oil
Add the milk, eggs, and vanilla. Whisk to combine.
1½ cups whole milk, 2 large eggs, 1 teaspoon pure vanilla extract
Whisk in the flour, cocoa powder, and baking powder, making sure to whisk just until no large lumps remain. You don’t want to ovemix.
2½ cups all-purpose flour, ½ cup unsweetened cocoa powder, 1 tablespoon baking powder
Add the mini chocolate chips and fold them into the muffin batter.
½ cup mini chocolate chips
Divide the batter between the muffin pans, filling each one completely full. Top with additional mini chocolate chips or coarse sugar, if desired.
Bake at 425°F for 5 minutes. Then, reduce the oven temperature to 350°F and bake for an additional 15 minutes, or until a toothpick inserted in the center comes out clean or with moist crumbs.
Remove the muffins to a wire rack and allow them to cool completely before storing.