Preheat the oven to 200c. Line two baking sheets with parchment paper.
In a large skillet over medium heat, melt the butter. Add the apples, and cook for a few minutes while stirring frequently. Add the brown sugar, lemon juice, cinnamon, and salt and stir to coat apple pieces. Cook for about 5 minutes or until the apples are softened and become syrupy. Reduce the heat to low.
Sprinkle in the cornstarch and stir until the juice boils and thickens, about 1 minute. Transfer to a bowl. Let cool for 15 minutes then refrigerate until chilled, about 30 minutes.
When the apples are chilled, roll each thawed puff pastry sheet out on a lightly floured surface into a 12-inch square. Cut each sheet into 4 equal squares.
