Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius) and line a baking sheet with parchment paper.

In a large bowl, cream the softened butter and granulated sugar until the mixture is light and fluffy.

Add the egg yolks and vanilla extract, beating until well combined.

Whisk together the flour and salt in a separate bowl, then gradually add to the wet ingredients, mixing until a dough forms.

Roll the dough into 1-inch balls and place them 2 inches apart on the prepared baking sheet.

Using your thumb or the back of a small round spoon, press a deep indentation into the center of each dough ball.

Fill each indentation with approximately 0.5 teaspoon of fruit preserves.

Bake for 12 to 15 minutes, or until the edges are just beginning to turn golden brown.

Cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.