Gently unfold the thawed puff pastry sheet on a lightly floured surface. Use a heart-shaped cookie cutter to cut out as many hearts as possible. You should be able to get about 12 hearts from one sheet. Place the heart-shaped pastry on a baking sheet lined with parchment paper.

Preheat your oven to 400°F (200°C). Bake the heart-shaped pastries for 12-15 minutes, or until they are golden brown and puffed up. Let them cool completely on a wire rack. This is crucial to ensure they crisp up nicely.

While the pastries are cooling, prepare the filling. In a large bowl, combine the heavy cream, powdered sugar, and vanilla extract. Use an electric mixer to whip the mixture until stiff peaks form. Be careful not to overwhip; you want it to be firm but still smooth.

Wash and hull the strawberries. Slice them thinly to make them easier to arrange on the tarts. If you are using larger strawberries, you might want to slice them in half or quarters to make them fit nicely on the heart shape.

Once the pastries are cool, gently slice each heart in half horizontally. Spread a generous layer of the whipped cream filling onto the bottom half of each heart. Arrange the sliced strawberries attractively on top of the cream.

In a small saucepan, heat the strawberry jam over low heat until it melts and becomes smooth. Brush the melted jam over the strawberries to give them a beautiful shine and a touch of extra sweetness. This step is optional but it will add a polished look to your strawberry heart tarts.