1. Heat oven to 400° F. Line 24 (1-3/4 inches in diameter) mini muffin cups with paper or foil baking cups.
2. Stir together flour, baking powder, sugar and orange rind in large bowl. Beat together in medium bowl yogurt, egg and melted butter. Add to flour mixture, stirring just until combined. Fold in chopped chocolate. Divide evenly into prepared muffin cups.
3. Bake 10 to 12 minutes or until well risen and golden brown.