Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
In a mixing bowl, combine the sliced strawberries, granulated sugar, cornstarch, and vanilla extract. Toss gently to coat the strawberries and set aside.
Lightly flour your working surface and roll out the puff pastry to about ⅛ inch thickness. Using a heart-shaped cookie cutter, cut out 16 hearts.
Place half of the hearts on the prepared baking sheet.
Spoon a generous amount of the strawberry mixture onto the center of each heart on the baking sheet, leaving space around the edges.
Brush the edges of each heart with the beaten egg to help seal them.
Place another heart-shaped puff pastry on top of the strawberry-filled ones and gently press the edges to seal.
Brush the top of each pastry with the remaining egg wash and make small slits on the top for steam to escape.
Bake for 15 minutes or until the pastries are golden brown and puffed up.
Allow to cool slightly before dusting with powdered sugar and serving.