Preheat oven to 375 degrees F. Line 36 1-3/4-inch muffin cups with paper bake cups; set aside. In a bowl, combine flour, granulated sugar, brown sugar, cocoa powder, baking powder, baking soda, and salt. Stir in chocolate pieces. Make a well in center of flour mixture; set aside.
In another bowl, whisk together oil, milk, and egg. Add oil mixture all at once to the flour mixture. Stir just until moistened. Spoon batter into prepared muffin cups, filling each two-thirds full.
Bake 12 to 15 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; cool. If desired, sprinkle with powdered sugar.