In a large mixing bowl, cream together the softened unsalted butter and granulated sugar until light and fluffy, about 3 to 5 minutes.

Beat in the egg yolk and vanilla extract until fully combined, about 1 minute.

In a separate bowl, whisk together the all-purpose flour and salt until well blended.

Gradually add the flour mixture to the butter mixture, stirring gently until a soft dough forms, about 2-3 minutes.

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

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

Scoop tablespoon-sized portions of dough and roll them into balls, arranging them on the baking sheet 2 inches apart.

Create an indentation in each cookie ball with your thumb or the back of a spoon.

Fill each indentation with a small amount of your chosen filling.

Bake for 12-15 minutes or until the edges are lightly golden.

Let the cookies cool on the baking sheet for a few minutes before transferring to a wire rack.

If desired, dust with powdered sugar before serving.