1. Preheat oven to 350°F. Lightly grease a 12 cup muffin pan.
2. In a large mixing bowl beat together butter, cocoa powder, sugar, baking powder, salt and vanilla until fluffy. Beat in eggs, then stir in milk and sour cream. Mix in flour, then chocolate chips.
3. Divide batter equally between the muffin cups, they will be full. Sprinkle each muffin with a little coarse sugar if desired.
4. Bake for 28-30 minutes, or until a toothpick inserted in center comes out without crumbs on it – chocolate is ok. Remove from oven, cool on wire racks 10 minutes, remove from pan to wire racks and cool completely.