In a medium saucepan, place the guavas and water and bring to a boil. Reduce heat to simmer and cook the guavas until very tender, about 10 minutes. Remove from heat. Place guavas and water in a blender or food processor and process. Strain the pulp and discard the seeds.
Return the pulp to the saucepan and cook with brown sugar, lemon juice, cinnamon stick, and salt over medium heat. Stir constantly with a wooden spoon until the mixture is thick. Remove from heat and cool completely.
Preheat oven to 400°F. Line two baking sheets with parchment paper.
Over a slightly floured surface, roll each sheet of puff pastry to a 12x12-inch square. Cut the sheets into 9 smaller squares. You will have 18 squares total.
Prepare an egg wash by whisking together 1 egg and 1 teaspoon of water. Brush the edges of each square with egg wash. Add 2 teaspoons of guava jam to the center of each square. Fold the pastry diagonally over the guava jam and seal each pastry by pressing with a fork.
Transfer pastries to prepared baking sheets and brush each pastry with egg wash. Sprinkle with granulated sugar. With a sharp knife, make a small slit on top of each pastry. Bake for 15 minutes or until golden brown. Serve warm.