Pre-heat the oven to 375°F.
Line a baking sheet with parchment paper; set aside.
Prepare the egg wash in a small bowl, by beating the egg with a pinch of salt; set aside.
On a lightly floured surface, roll one sheet of puff pastry into an 8 x 12-inch rectangle; roughly the size of a standard sheet of paper; keep the other puff pastry sheet in the refrigerator.
Dust off any excess flour and lay the rolled pastry sheet on the parchment-lined baking sheet.
Dip a pastry brush into the egg wash, and paint a line in lengthwise along the center of the pastry sheet, followed by three crosswise lines, evenly spaced, forming a grid of 8 rectangles, each approximately 3-inches wide and-4 inches long.
Brush edges with egg wash.
Lay a slice of guava paste in the center of each rectangle.
Spread some of the cream cheese, if using, on top of the guava paste; about 1-1/2 tablespoons cream cheese per pastry.
Remove the second sheet of puff pastry from the refrigerator, and roll it out into an 8 x 11-inch rectangle, dusting with flour as needed to prevent sticking.
Lay the second sheet of puff pastry on top of the first.
Using the side of your palm, press around and between the mounds of filling, pressing out any excess air, and to glue the top pastry to the bottom, creating 8 even, rectangular pockets.
Paint the top sheet of puff pastry with egg wash.
Sprinkle the surface with sugar.
Using a knife or bench scraper, cut out and separate the 8 pastelitos following the original grid in between the indentations in the dough.
Trim the perimeter to neaten the rectangles.
The dough should still be cool to the touch; if it is warm, return the pastelitos to the refrigerator on the baking sheet for 10 minutes to firm up before baking.
Bake pastelitos until puffed and golden brown; about 20 minutes.
Serve warm or at room temperature.