Special equipment: a 3-inch round cookie cutter or a clean 3-inch diameter can with the top removed
Preheat the oven to 350 degrees F. Spray a large baking sheet with nonstick cooking spray. Unfold 1sheet of puff pastry on lightly floured surface. Using a 3-inch cookie cutter or a 3-inch diameter can with the top removed, cut out 9 circles of dough. Repeat with the remaining puff pastry sheet. Put a scant teaspoon of cajeta in center of each round. Top each with a sprinkling of grated cheese and 3 or 4 pieces of pineapple. Brush the edge of each circle with the egg mixture and enclose each empanada into a half moon. Crimp the edges with fork. Transfer to prepared baking sheet and brush each empanada with the egg mixture. Crimp edges with fork.Transfer the empanadas to the prepared baking sheet and brush them with the egg mixture. Sprinkle with demarara sugar and bake until golden brown, about 25 to 30 minutes. Transfer the empanadas to a serving platter and serve warm.