Crust- Whisk flour, sugar and salt. Add cold butter and mix with pasty blender or fork until mixture resembles coarse meal and the pieces of butter are pea sized. Add 1 tbsp cold water and continue mixing. Continue adding water 1 Tbsp at a time until dough holds together when you pinch it. (you may not need all the water). Wrap dough tightly in plastic wrap and refrigerate at least 2 hours.
Filling- Preheat oven to 400. Line 2 baking sheets with parchment paper. Cut apples into small pieces(about 1/2 inch). Placed chopped apples and butter in large pan over medium heat. Cook for 3-4 minutes, stirring frequently. Add sugar and cinnamon and continue cooking for another 3-4 minutes.
Assemble- Remove chilled dough from refrigerator and place on a clean, well floured surface. Roll out dough to about 1/4 inch thickness. Using sharp knife, cut dough into 4 inch squares. Assemble dough scraps and re-roll as needed. Place dough squares on lined pans. Spoon 1 & 1/2 teaspoons apples mixture into center of each square. Fold corners toward center, overlapping corners slightly on top and pinching them together.
Brush top of each square with melted butter. And sprinkle top with 1/4 teaspoon cinnamon-sugar.
Bake one pan at a time, 14-16 minutes or until crust is golden brown. Cool slightly on pans on wire racks.