Preheat your oven to 400°F (200°C). Line the baking sheet with parchment paper.

In a mixing bowl, combine the diced strawberries, granulated sugar, and vanilla extract. Allow this mixture to sit for about 10 minutes, so the strawberries release some juice.

On a floured surface, roll out the puff pastry sheet to smoothen any creases. Cut the pastry into heart shapes using a sharp knife or pastry cutter. You should be able to cut out 8 hearts.

Take 4 of the heart shapes and place a spoonful of the strawberry mixture in the center of each. Be careful not to overfill.

Lightly brush the edges of the hearts with the egg wash (made by whisking the egg and milk together). This will help seal the pastries.

Place the remaining 4 heart shapes on top of the filled hearts, pressing the edges to seal. You can use a fork to crimp the edges for a decorative touch.

Brush the top of each pastry with the remaining egg wash for a golden finish.

Place the hearts on the prepared baking sheet and bake for 15 minutes, or until they are puffed and golden brown.

Once baked, allow them to cool slightly before dusting with powdered sugar (optional) and serving.