Preheat the oven to 180°C / 350°F (top/bottom heat). Line a baking sheet with parchment paper.
Peel and finely dice the apple. Melt the butter in a pan, add apple, sugar, cinnamon, and lemon juice, and cook for 2–3 minutes. Mix the cornstarch with the water, stir it in, and cook for 1 more minute until lightly thickened. Let the filling cool completely.
Unroll the puff pastry and cut it in half crosswise, creating 2 long strips..Cut triangles from both strips (base approx. 7 cm / 2.7 in wide).
For each croissant, place 3 triangles slightly overlapping, pressing the seams gently. Place 1 heaping teaspoon of apple filling onto the wide end.
Roll up tightly from the wide end and place the croissant on the tray with the tip facing down.
Brush the tops with the egg yolk–milk mixture.
Bake for 18–22 minutes until golden and puffed.
Let cool slightly and dust lightly with powdered sugar.
