Preheat the oven to 375°. In a small bowl, whisk the egg yolk with the milk.
On a lightly floured work surface, unfold the puff pastry and cut it into 6 squares. Transfer the squares to a parchment paper-lined baking sheet. Cut the guava paste into 6 equal portions and spoon onto half of each pastry square. Top with the cream cheese. Brush the pastry edges with some of the egg wash. Fold the pastry over the filling to form rectangles and crimp the edges with a fork. Refrigerate for 15 minutes, until firm.
Brush the pastries with the remaining egg wash and sprinkle with sugar. Bake in the center of the oven for 30 minutes, rotating the pan from front to back halfway through, until the pastries are golden. Let cool for at least 30 minutes before serving.