Preheat your oven to 425°F and line a baking sheet with parchment paper.
In a medium pot over medium heat, melt the butter and add the chopped apples, brown sugar, lemon juice, cinnamon, and salt.
Stir the mixture and cook until the apples are soft and most of the juice is gone, which will take about 10 minutes.
Stir in the flour and cook for another 30 seconds, then remove from heat and let the filling cool in the refrigerator for at least 15 minutes.
Roll out the puff pastry and cut it into 8 squares. Spoon the cooled apple filling onto each square.
Brush the edges of the pastry squares with water, fold them over, and crimp the edges with a fork to seal.
Brush the tops with the beaten egg wash and cut three small slits on each top for venting.
Mix the remaining sugar and cinnamon together and sprinkle the mixture over each pastry. Bake for 15 minutes or until golden brown.