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

Cream together the softened unsalted butter, granulated sugar, and light brown sugar until light and fluffy.

Incorporate the egg yolk and vanilla extract into the creamed mixture until evenly combined.

Gradually add the all-purpose flour to the mixture and combine gently until just incorporated.

Shape the dough into one-inch balls and place them on the prepared baking sheets with space in between.

Press your thumb or a teaspoon into the center of each dough ball to create indents and fill with jam.

Bake for 12-15 minutes or until the edges are lightly golden; cool on baking sheets before transferring.