Prep - Thaw frozen puff pastry for 40 minutes at room temperature. Beat egg and milk to make an egg wash. Preheat oven to 400°F (205°C).
Prepare Fillings - In a bowl, whisk cream cheese and then whisk in vanilla and powdered sugar. Cut guava paste into 16 ¼-inch square or rectangular slices.
Assemble - Lay one sheet of puff pastry on top of a parchment lined baking pan. Make a 4 x 4 grid, with each box stacked with guava with cheese spread over in the middle. Leave a 1-inch space between stacks. Top with the second puff pastry sheet. Press down gently around fillings and then using a pizza cutter, slice pastry between filling into 16 squares. Working quickly, crimp all 4 edges of each to seal.
Bake - Arrange pastelillos on the baking pan evenly with space between each. Brush to tops and sides with egg wash. If using granulated sugar, now is the time to sprinkle sugar over some-or-all of them. Bake until pastelillos are a gorgeous golden brown, flaky, and puffy, about 25 minutes. Remove from oven and let them cool for at least 10 minutes.
Serve - Once they've cooled, sprinkle with generous amounts of powdered sugar. Don't add powdered sugar to any bakes with granulated sugar as they're already sugared. Pastelillos are perfect for a midday snack or dessert with coffee or milk.