Preheat oven to 400°.Line a large baking sheet with parchment paper and set aside.
Transfer 10 ounces guava pastein 20-30-second increments until it's loose enough to stir. Set aside.Place 8 ounces cream cheese in another bowl and stir until creamy.
In a small bowl, beat 1 large egg and 1 tablespoon water with a fork. Set aside.
Roll out 17 ounces puff pastry sheetsstry sheets into a large square and use a pizza cutter or sharp knife to cut the pastry into nine equal squares.
Spread a bit of cream cheese into the center of each square, leaving about ½" border around the perimeter of the pastry. Top the cream cheese with a bit of guava paste.
Dip a pastry brush into the egg wash along the rim of each pastry square. Fold the pastry over the filling and press along the edges with the tines of a fork to seal. Use a sharp knife to make three slits on the top of the pastry so steam can escape. Transfer the pastries to the baking sheet. Continue in this manner with each pastry.
Brush the pastries with the rest of the egg wash and sprinkle the tops with ¼ cup demerara sugar. Bake for 10 minutes, flip the baking sheet 180° and continue to bake for 8-10 minutes or until golden brown. Remove from oven and let cool for 10 minutes before serving.