1. In a large bowl, whisk together eggs and egg whites until well blended. Whisk in milk and set aside. In a medium bowl, combine flour, sugar and salt. Gradually add flour mixture to egg mixture, stirring well with a whisk. Let stand 30 minutes.
2. While batter is standing, preheat oven to 450 degrees. Cook bacon strips until crispy. Drain, let cool, and finely chop.
3. Coarsely grate apple, including peel. Coat a 9-cup popover pan with cooking spray. Fold the bacon, grated apple, and melted butter into batter, stirring until well blended.
4. Pour batter into prepared popover cups until ¾ full. Bake at 450 degrees for 18 minutes.
5. Reduce heat to 325 degrees and bake for 10 minutes more. Serve immediately with apple butter.