Preheat the oven to 400 °F (200 °C).
Flour a flat surface and lay the puff pastry on it.
Flour the top of the pastry and roll it out thin.
Using a cookie cutter or any circular object, cut out 20 to 24 circles, and brush the edges of each one with the beaten egg. The egg is what seals them.
In the centre of half the circles, place a thin slice of guava paste, a thin slice of cheese and, if using, half a tablespoon of arequipe. Keep the filling thin: overfilled pastries force themselves open in the oven.
Place a second circle on top of each to enclose the filling.
Seal the edges by pressing firmly all the way round with your fingers or a fork. Repeat with the rest.
Place them on a tray lined with parchment paper, leaving space between them so they can puff.
Bake for about 20 minutes, until puffed and golden.
Take them out and sprinkle with granulated sugar while they are still hot, so it sticks.
Let them cool slightly and serve warm or at room temperature. The filling is very hot straight from the oven.