Preheat the oven to 400°F (200°C).
Roll out the puff pastry on a lightly floured surface.
Cut the pastry into squares (about 4x4 inches).
Prepare the strawberry filling by mixing the chopped strawberries, sugar, cornstarch, and vanilla extract in a bowl.
Spoon a dollop of the strawberry filling onto the center of each pastry square.
Fold the corners of the pastry squares toward the center, pinching to seal.
Brush the tops of the pastries with beaten egg for a golden finish.
Bake in the preheated oven for about 20 minutes or until golden brown.
Let the pastries cool before serving.