Preheat the oven to 400-degrees F, and line a large baking sheet with parchment paper.
Add the apples, cinnamon, lemon juice, sugar, butter and salt to a saute pan, and cook over medium heat until the apples soften and caramelize. Stir frequently.
Remove the apples from the heat and let cool to room temperature.
Take the puff pastry out from the fridge.
Sprinkle the work surface and puff pastry with flour. Roll out the dough into a 12-inch square. Use flour to prevent sticking. Cut the dough into four equal squares.
Divide the apple mixture into 4 piles in the corners of the dough, but leave about 3/4" of border. Fold the edges of the dough over each of the apple piles. You will have 4 triangular pies.
Crimp tightly along the edges with a fork to seal.
Move the pies to the baking sheet. Brush each pie generously with the beaten egg yolk.
Bake the pies for 20-25 minutes, or until golden brown and puffy.
Flaky, buttery and sweet.
