Heat oven to 400F. Line a baking sheet with parchment paper and cut 1/4 inch thick slices of the guava paste. Add the egg to a small bowl, beat, then set aside.
Add the cream cheese, and salt to a stand mixer fitted with a paddle attachment or a large bowl if using an electric hand mixer.
Cream the cheese for a minute then add in the powdered sugar and vanilla extract. Mix until well combined then scrape the bowl down and transfer to a piping bag, or a plastic bag, with the tip snipped off.
On a lightly floured surface roll the thawed puff pastry out to about 1/8 an inch. Cut into roughly equal squares or rectangles. Repeat the process for the second pastry sheet.
Transfer six of the squares to a parchment-lined baking sheet then pipe a thin layer of the cream cheese mixture onto each square leaving about 1/2 an inch of space from the edge. Place the guava paste slices on top of the cream cheese mixture cutting as needed to maintain the clear edge.
Brush the edges with the egg wash then top with another square of puff pastry. Press the edge together then use a sharp knife to cut the edges and clean them up. This is optional but it helps seal the pastelitos and gives them a nice presentation.
Brush the top with the egg wash then sprinkle with turbinado sugar. You can score the top if desired.
Bake at 400F for about 15 minutes or until golden brown and puffed up.