Preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a mixing bowl, cream together the softened butter and sugar until light and fluffy. Add the egg yolk and vanilla extract, mixing until combined.

Gradually add the flour while mixing on low speed until a soft dough forms.

Divide the dough in half. Roll out one half on a floured surface to about 1/4 inch thick. Use heart-shaped cookie cutters to cut shapes, cutting out centers from half of them.

Place half of the hearts on the prepared baking sheets. Spoon about half a teaspoon of jam into each center cut-out before placing another cookie on top.

Bake for 12-15 minutes or until edges are golden brown. Allow cooling on wire racks before serving.