Cream together the softened unsalted butter and granulated sugar until light and fluffy, about 3 minutes. Beat in the egg yolk and vanilla extract until well combined.

Whisk together the all-purpose flour and salt in a separate bowl. Gradually add to the butter mixture, stirring until a soft dough forms. Cover and refrigerate for at least 30 minutes.

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

Scoop dough into balls, placing them 2 inches apart on the baking sheet. Press a thumbprint into each and fill with fruit preserves or chocolate ganache.

Bake for 12-15 minutes until edges are lightly golden. Let cool for about 5 minutes before transferring to a wire rack.

Optional: Dust with powdered sugar once cooled for added sweetness.