Preheat oven to 375 degrees F. Line twelve 2 1/2-inch muffin cups with paper bake cups; set aside. In a medium bowl stir together flour, granulated sugar, brown sugar, cocoa powder, baking powder, baking soda, and salt. Stir in chocolate pieces. Make a well in the center of flour mixture; set aside.
In a small bowl combine egg, milk, and oil. Add egg mixture all at once to flour mixture. Stir just until moistened. Spoon batter into the prepared muffin cups, filling each about two-thirds full.
Bake for 18 to 20 minutes or until edges are firm (tops will be slightly rounded). Cool in muffin cups on a wire rack for 5 minutes. Remove from muffin cups; serve warm.