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

In a mixing bowl, cream together softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the egg yolk and vanilla extract; mix until well combined.

Gradually incorporate the all-purpose flour until just combined.

Roll tablespoon-sized portions of dough into balls and place them on the prepared baking sheets.

Press your thumb gently into the center of each ball to create an indent, then fill with about half a teaspoon of fruit jam.

Bake for 12-15 minutes or until lightly golden.