1. Heat oven to 350 °F.
  2. Grease 24 3-inch muffin pan cups or line with paper liners.
  3. In large bowl, combine flour, 1 cup sugar, the baking powder, salt, orange zest, and nutmeg.
  4. With pastry blender or fork, cut in ¾ cup (1½ sticks) butter until mixture is crumbly.
  5. In medium bowl combine apple, yogurt, and eggs; add to flour mixture, stirring until just combined.
  6. Spoon batter into prepared muffin cups and bake 20 to 25 minutes or until centers spring back when gently pressed.
  7. Cool muffins in pan 5 minutes; remove from pan and cool until able to handle.
  8. Meanwhile, melt remaining ½ stick butter.
  9. In small bowl, combine remaining ¾ cup sugar and the cinnamon.
  10. To serve.
  11. Brush tops of muffins with butter and roll top in cinnamon-sugar mixture.
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