Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a mixing bowl, combine the sliced strawberries, granulated sugar, and vanilla extract. Mix gently and let it sit for about 5-10 minutes until the strawberries release some juice.
Lightly flour your work surface and roll out the puff pastry sheet to smoothen it (if necessary). Cut out heart shapes using the cookie cutter. You should get about 8 hearts.
Place 4 of the heart shapes on the prepared baking sheet.
Spoon a small amount of the strawberry mixture onto the center of each heart on the baking sheet, leaving a small border around the edges.
Top each filled heart with another heart shape, pressing the edges lightly to seal. You can crimp the edges with a fork for an added decorative touch.
In a small bowl, whisk the egg and brush it over the top of each pastry heart for a golden finish.
Bake for 15-20 minutes or until the pastries are puffed and golden brown.
Once baked, remove from the oven and allow to cool slightly. Dust with powdered sugar before serving.