Preheat oven to 400°F (200°C) and line a baking sheet with parchment.
Toss diced apple with brown sugar, lemon juice, and cinnamon. Let sit 5 minutes to soften.
Place a spoonful of apple mixture and a sprinkle of cheddar in the center of each pastry square.
Brush edges with melted butter, fold into triangles or rectangles, and press edges with a fork to seal.
Brush with egg wash and bake 15–18 minutes, until golden brown.
Brush with maple syrup as soon as they come out of the oven and let cool slightly before serving.