Preheat oven to 400 degrees F. Line a baking sheet with parchment paper.

Flour a clean, dry counter top or other flat working surface. Place the puff pastry sheet on the work surface and roll out into a rectangle. Cut the rectangle in half lengthwise and then cut each half crosswise into 3 squares. Place the squares on the prepared baking sheet.

Brush a ½-inch border of the egg around 2 adjacent sides of each pastry square.

Spoon arequipe in the middle of the square. Fold the pastry over the filling to make a triangle and press the edges with your fingers or a fork to seal.

Repeat with the remaining squares. Brush the tops with the remaining egg and sprinkle with the sugar. Bake until the pasteles are golden brown, about 15 minutes. Let cool slightly on the baking sheet, then serve warm.