Preheat your oven to 400℉. (or whatever your puff pastry package suggests)
Peel, core and dice the apples into 1 cm cubes (roughly)
In the pot, melt the 2 Tbsp butter, melted then add the 3 tsp ground cinnamon
Once melted and combined, add in the apples that have been diced or sliced (if you prefer), and the ¼ cup granulated sugar and 2 Tbsp water
Cover and cook over medium heat for 4 to 6 minutes until the apples have softened a bit (you don't want them too mushy)
In a small bowl combine the 1 Tbsp cornstarch and the 1 Tbsp water (for the slurry) to make a slurry then add to the apple mixture and cook until the sauce is thickened and the apples are tender but not mushy. Let cool
While the mixture is cooling, roll out your puff pastry on a floured surface
If you have 2 sheets, roll out one sheet to 1/8" thickness and cut it into 4 equal squares, set aside and repeat with the second sheet. You should end up with 8 squares that are each about 6" square
line a baking sheet with parchment paper
lay 4 squares of puff pastry on the baking sheet, spacing them apart by at least 2-3"
Once the filling has cooled, place about 1/2 cup of filling on the center of each square
using an egg wash (1 egg with 1/2 Tbsp water, beaten), going one square at a time, brush the entire edge of the bottom pastry (about 1/2" in) with the egg wash then place another square of pastry on top, centered over the bottom piece. Lightly press down the edges to seal the pastry removing any air bubbles as you go, then go over the edge with a fork, lightly pressing to ensure it has sealed. Repeat with remaining 3 pastries.
Lightly brush the top of each pastry pocket with egg wash, then sprinkle on some decorating surgar (or regular granulated surgar) to finish the pies off.
Bake in preheated oven at 400℉ for 25 minutes or until golden brown
Remove from oven, let sit at least five minutes. Filling will be HOT! Enjoy