Heat oven to 375ºF.  Grease muffin cups generously or use foil baking cups. Finely grated orange peel. Try to take just the orange part. A little white pith will probably adhere. Using a wooden spoon or electric mixer, beat sugar and butter in large bowl until pale and fluffy. Beat in eggs, one at a time. Add the orange peel. Add yogurt, orange juice, baking powder, baking soda and chocolate chips; mix well. Batter will be quite liquid. Sprinkle flour over batter. Fold gently, just enough to blend in the flour. Scoop the batter into the muffin cups. Fill the cups only 3/4 full since the batter rises. Bake 15-25 minutes or until golden brown and springy to touch. Turn out onto rack to cool. This makes lots of batter for 10 large muffins or 15 regular size muffins or 36 mini-muffins.