Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper. Working with 1 sheet of puff pastry at a time, unfold on a lightly floured surface and cut out four 4-inch circles.
Beat the egg with 1 tablespoon water in a small bowl. Place 1/2 teaspoon cajeta in the center of each circle. Top each with some grated cheese and a few pieces of pineapple. Brush the edge of each circle with the egg wash and fold in half to form a half-moon. Crimp the edges with fork.
Transfer the empanadas to the prepared baking sheet, brush with the remaining egg wash and sprinkle with the demerara sugar. Bake until golden brown, about 30 minutes.