Chocolate Cheese Muffin

1/2 cup (113 grams) unsalted butter, melted and cooled
2 large eggs
1 cup (240 ml) buttermilk
2 teaspoons pure vanilla extract
1 3/4 cups (230 grams) all-purpose flour
2/3 cup (60 grams) unsweetened cocoa powder
1 1/4 cups (265 grams) light brown sugar
1 teaspoon baking powder
1 teaspoon baking soda
1/2 teaspoon salt
Preheat oven to 375 degrees F (190 degrees C). Position rack in center of oven. Butter, or line with paper liners, 12 - 2 3/4 x 1 1/2 inch muffin cups.
In a large measuring cup or bowl whisk together the eggs, buttermilk, and vanilla extract. 
In another large bowl whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt. Stir in the chocolate chips. With a rubber spatula fold the wet ingredients, along with the melted butter, into the dry ingredients and stir only until the ingredients are combined. Do not over mix the batter or tough muffins will result. 
Evenly fill the muffin cups with the batter (the muffin cups will be full), using two spoons or an ice cream scoop. Place in the oven and bake until a toothpick inserted in the center of a muffin comes out clean, about 20 minutes. Transfer to a wire rack and let cool for about 5 minutes before removing from pan.