Thaw the puff pastry according to its package instructions. Keep it refrigerated until ready to shape. Soften the cream cheese just enough to mix easily.
Position oven racks in the upper and lower thirds of the oven. Preheat to 400°F (200°C) using a conventional oven setting. Line two large baking sheets with parchment paper.
Mix the cream cheese, 2 tablespoons sugar, and vanilla in a bowl until smooth. Avoid whipping in excessive air.
Divide the guava paste into 18 equal portions, approximately 12–13 g each. Cut each portion into a thin strip about 2½ inches long, or use several small pieces arranged in a short line.
Beat the egg with 1 tablespoon water in a small bowl to make the egg wash.
Lightly flour the work surface. Gently roll one cold pastry sheet into an approximately 12-inch square. Cut it into nine equal 4-inch squares. Repeat with the second sheet. If the pastry becomes soft or sticky, refrigerate it briefly before continuing.
Arrange each square like a diamond. Divide the cream cheese mixture evenly among the 18 squares, using approximately 14 g per pastry. Spread it in a short line from the upper corner toward the lower corner, leaving about ½ inch clear at both ends.
Place one portion of guava paste over the cream cheese on each square. Keep the filling narrow so the side corners can overlap comfortably.
Fold the left corner over the filling. Lightly brush the overlapping area with egg wash, then fold the right corner over it. Press the overlapping pastry firmly together. Leave the two ends slightly open.
Transfer the pastries to the prepared baking sheets, leaving at least 2 inches between them. Refrigerate for 15 minutes to firm the dough.
Brush the tops lightly with egg wash, avoiding drips down the exposed cut edges. Discard any unused egg wash.
Bake at 400°F (200°C) for 20–25 minutes, until puffed and golden with visibly flaky edges and golden bases. Rotate the trays and swap their positions halfway through baking. If baking one tray at a time, keep the second tray refrigerated until needed.
While the pastries bake, combine the glaze ingredients—2 tablespoons sugar and 2 tablespoons water—in a small saucepan. Bring to a gentle simmer, stirring until the sugar dissolves, then remove from the heat.
Brush the freshly baked quesitos lightly with the sugar glaze. Use only enough for a thin shine; excess syrup can soften the crisp pastry.
Let the quesitos cool for at least 15 minutes before serving. Serve warm rather than piping hot, because the guava filling retains heat.