Heat oven to 425. Line a baking sheet w/ parchment paper. Unfold pastry onto a clean surface, cut into 4 equal squares.
Melt butter in lg skillet, over mediu heat, stir in apples, 1/4 cup sugar, cinnamon and cornstarch. Cook, stirring for 4 mins. Spoon 2 tbsp apple mix into center of each pastry square.
In small bowl, whisk together egg white and water. Brush edges of each pastry square w/ egg wash. Fold squares into triangles. Press edges w/ fork to seal. Brush tops w/ egg wash. Place on baking sheet. Sprinkle w/ sugar. BAke 12-14 mins or until lightlt brown.