Cream together the butter and granulated sugar until light and fluffy. Add the egg yolk and vanilla extract, mixing until smooth.

In a separate bowl, whisk together the flour and salt. Gradually add the dry ingredients to the wet mixture, mixing until a soft dough forms.

Cover the dough with plastic wrap and refrigerate for at least 30 minutes.

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

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

Make an indentation in the center of each ball using your thumb.

Fill each indentation with your choice of fruit preserves or chocolate ganache.

Bake in the preheated oven for 12-15 minutes until lightly golden around the edges.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack.

If desired, dust the cooled cookies with powdered sugar before serving.