Preheat oven to 400℉ (205℃)
Cut puff pastry sheet into six 5" x 5" squares (if needed, gently roll the puff pastry so it is large enough to cut out the squares).
1 Sheet Puff Pastry
Peel and core apple. Cut apple into six ¼" slices.
1 Large Apple
Combine the cinnamon and sugar in a shallow dish. Dip both sides of each apple slice in the mixture.
1 teaspoon Ground Cinnamon, ⅔ Cup Granulated Sugar
Place an egg in a bowl and whip.
1 Large Egg
Brush puff pastry with the whipped egg. Cut four slits in puff pastry square (the cuts will be made slightly rounded on each corner). Place an apple slice in the middle of each square.
Fold opposite ends of puff pastry over apple slice. Using fingers, seal where the puff pastry meets in the middle.
Fold remaining opposite ends of puff pastry over the apple slices. Using fingers, seal where the puff pastry meets in the middle. Fold remaining dough edges over apple slice and press lightly with fingers to seal.
Brush the pastry with egg wash and sprinkle with cinnamon and sugar.
Place pastries 2" apart on a non-stick baking sheet.
Bake for 12 - 15 minutes.
Cool for 10 minutes on the baking sheet then transfer to a wire rack to cool completely.