Preheat oven to 400°.
Combine first 8 ingredients in a large bowl; make a well in center of mixture. Combine apple, buttermilk, oil, and egg in a bowl; stir well. Add to dry ingredients, stirring just until moist.
Divide batter evenly among 12 muffin cups coated with cooking spray. Combine remaining ingredients; sprinkle over muffins. Bake at 400° for 20 minutes or until golden. Remove from pans immediately. Serve warm.