Preheat oven to 350 F. Line muffin pans with paper liners or spray-coat with baking spray. Cream butter and sugar in a large bowl. Add eggs and bananas; mix well.
Combine flour, baking powder and salt in a separate bowl. Mix sugar with cinnamon in a small bowl; coat apples with cinnamon-sugar mixture. Gradually add flour mixture and milk to butter mixture, beating on medium speed until combined. Stir in apples and vanilla. Spoon batter into muffin cups until half full. (If desired, sprinkle additional cinnamon and sugar on top.) Bake for 20 minutes. Cool for 10 minutes on a wire rack. Store in an airtight container for up to 4 days.
Makes about 30 muffins