Heat oven to 400°. Spray bottoms only of mini muffin pan cups with cooking spray. For topping, combine sugar and cinnamon in small bowl; mix well and set aside. For muffins, combine all-purpose flour, whole-wheat flour, oats, brown sugar, baking powder, baking soda, salt, and cinnamon in a large bowl; mix well. In small bowl, combine apple juice, egg, oil and vanilla; blend well. Add to dry ingredients all at once; stir just until dry ingredients are moistened. (Do not overmix.) Fill muffin cups two-thirds full. Sprinkle topping evenly over tops of muffins. Bake 14 to 18 minutes or until light golden brown. Cool muffins in pan on wire rack 5 minutes; remove from pan. Serve warm. Makes 3 dozen.
For regular-size muffins: Line 12 medium muffin pan cups with paper baking cups or spray bottoms only with cooking spray. Prepare muffin batter as directed above. Fill muffin cups almost full. Sprinkle with topping. Bake 18 to 22 minutes or until light golden brown. Cool muffins in pan on wire rack 5 minutes; remove from pan. Serve warm.