1. Melt butter over medium-high heat in a medium skillet. Add apples and cook, stirring occasionally, until browned and beginning to soften, 5 minutes. Stir in 2 Tbsp. sugar and maple syrup. Cook, stirring, until apples are soft but not falling apart and liquid is syrupy, 1 to 2 minutes. Transfer to a bowl and let cool completely.
2. Preheat oven to 400ºF. Line a baking sheet with parchment. Unroll pastry; cut into 4 squares.
3. Place 1 piece of pastry in front of you so that a point faces you. Spoon apple mixture over half, leaving a 1/2-inch border. Fold over to enclose filling, forming a triangle. Press edges with a fork to seal. Repeat with remaining pastry pieces and apples. Place on baking sheet, brush with milk, sprinkle with remaining 1 Tbsp. sugar and bake until puffed and golden, 20 to 22 minutes. Let stand for 5 minutes. Serve warm.