Preheat your oven to 400°F (200°C).

In a medium bowl, combine the diced strawberries, granulated sugar, cornstarch, and vanilla extract. Stir gently.

Roll out the puff pastry sheets on a lightly floured surface and cut each sheet into four squares.

Place a spoonful of the strawberry mixture in the center of each pastry square.

Fold each square into a triangle, pinching the edges to seal them tightly.

Brush the tops with the beaten egg.

Place the turnovers on a parchment-lined baking sheet and bake for about 15-20 minutes, or until puffy and golden brown.

Let them cool before dusting with powdered sugar.