Beat the egg with a fork in a small bowl, then whisk with water (approximately ½ to 1 tablespoon) and lightly beat to create an egg wash. Set aside to use later.
Using a stand mixer, combine goat cheese with sugar and vanilla and purée until smooth.
In a separate bowl, mix the guava jam with lemon juice until smooth.
Put a baking sheet in oven to preheat for five minutes while you prepare the pastry.
Roll out pastry on a floured work surface with a floured rolling pin into a larger rectangle (but smaller than the sheet pan you're using). Repeat this process with the other sheet of pastry, making it slightly larger than the first piece (to be able to cover the edges later on).
Transfer the first piece of pastry to a sheet of parchment paper or foil. Spread the goat cheese mixture evenly over the pastry, leaving 1-inch borders on all sides. Dollop the guava jam on top so it evenly covers the goat cheese mixture, remembering to leave a 1-inch border around the outside.
With the second sheet of pastry, use a sharp knife to cut slashes, designs, or other air holes in the top, making sure to leave a 1-2-inch border around the sides to prevent filling spillage.
Brush some egg wash around filling on the bottom layer. Lay the second, cut pastry over the filling on the bottom layer, lightly pressing the edges to seal and brushing the top of the dough with more egg wash.
Bake on a hot baking sheet until puffed and golden-brown, ~20 to 30 minutes (may be 5-10 minutes difference depending on your oven settings). Transfer pastry to a rack to cool completely. This pastry will last about a week in the fridge, sliced in a Tupperware container or on a plate in a ziplock bag.