Preheat oven to 350°F. Lightly grease a 12 cup muffin pan or line with muffin liners.
In a large bowl beat together butter, cocoa powder, sugar, baking powder, salt, and vanilla until fluffy. Beat in eggs, then mix in milk and sour cream. Mix in flour, then chocolate chips.
Divide batter equally between the muffin cups; they will be full. Sprinkle each muffin with a little coarse sugar if desired.
Bake for 25-30 minutes, or until a toothpick inserted in center comes out without crumbs on it. (Some chocolate on the toothpick is ok.)
Remove from oven and cool on a wire rack for 10 minutes. Remove muffins from the pan and set on wire racks to cool completely.
Notes
Recipe Variations & Substitutions:
You can use 2 cups all-purpose flour or 2 cups whole wheat pastry flour instead of using two different flours.
I like using whole milk for the higher moisture content, but you can substitute a lower-fat milk or a nondairy milk.
Plain yogurt can be used in place of the sour cream.
Chocolate Flavor: These muffins are an excellent choice for birthdays and special occasions when you want a chocolate treat but want to be just a bit healthier!