Preheat oven to 375. Butter and flour one 12 cup and one 6 cup muffin pan or line with foil or paper liners. Mix flour, baking powder, baking soda, and salt in large bowl. Beat sugar, egg, milk, oil, orange zest, and vanilla with mixer on high speed until well mixed. Stir orange mixture into dry ingredients. Stir in chocolate chips by hand. Spoon batter into prepared cups, filling each one two thirds full. Bake until a toothpick inserted into the center comes out clean, 20 to 25 minutes. Cool muffins on racks. Makes about 18 muffins.
The Golden Book of Chocolate