Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
On a lightly floured surface, roll out the puff pastry sheet to smooth it out and slightly enlarge it. Cut into 4 equal squares.
In a medium bowl, combine the softened cream cheese, powdered sugar, and vanilla extract. Mix until smooth and creamy.
Place a generous dollop of the cream cheese mixture in the center of each puff pastry square.
In another bowl, mix the fresh blueberries with the lemon juice and distribute them evenly over the cream cheese mixture on each square.
Fold the corners of each pastry square towards the center, pinching to seal but leaving the center exposed.
Brush the edges of each pastry with the beaten egg to give it a golden finish when baked. Sprinkle coarse sugar over the top for added sweetness and crunch.
Bake in the preheated oven for 15-20 minutes, or until the pastries are puffed and golden brown.
Allow to cool slightly before serving.
Notes
Serve warm on a decorative plate, garnished with a few extra blueberries and a light dusting of powdered sugar for elegance.