In a large mixing bowl, using an electric mixer, cream together the butter and sugar until the mixture is light and fluffy, which should take about 2-3 minutes.

Beat in the egg yolk and vanilla extract to the creamed butter and sugar mixture until everything is well combined and smooth.

In a separate bowl, whisk together the flour and salt. Gradually add this dry mixture to the wet ingredients, stirring gently until the dough just comes together without overmixing.

Wrap the dough tightly in plastic wrap and place it in the refrigerator for at least 30 minutes.

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

Once the dough has chilled, scoop tablespoon-sized portions and roll them into balls, placing them on the prepared baking sheet about 2 inches apart.

Using your thumb or the back of a spoon, create an indentation in the center of each dough ball.

Spoon a small amount of fruit preserves or chocolate ganache into each indentation.

Place the baking sheet in the preheated oven and bake for 12-15 minutes, or until the edges of the cookies are lightly golden.

Allow the cookies to cool on the baking sheet for a few minutes before gently transferring them to a wire rack to cool completely.