Unfold the defrosted (but still refrigerator cold) puff pastry sheet on a piece of parchment paper
Roll out the puff pastry sheet in both directions to create a slightly flatter and even surface
Evenly cover the surface of the puff pastry with the jam
Fold the long sides of the puff pastry towards the center so they go halfway to the middle. Fold them again so the two folds meet exactly at the middle of the dough. Then fold 1 half over the other half.
Put the folded puff pastry in the freezer for 30 minutes to allow the butter to harden before cutting
Preheat your oven to 350°F (180°C)
Slice the dough into 3/8-inch slices, this should yield approx. 25 hearts
Bake for 15-20 minutes. The hearts are done baking when they're golden and will continue to harden as they cool
Transfer hearts to a cooling rack
Store in an airtight container
1 package (17.3 ounces) of Puff Pastry sheets, contains 2 sheets. Double the recipe to use one package. You can use a jam of your choice or try Nutella for Chocolate Hearts.