Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Cream together the softened butter and sugar until light and fluffy.
Add the egg yolk and vanilla extract, mixing until well combined.
Gradually sift in the flour and salt, mixing until a dough forms.
Chill the dough in the refrigerator for about 30 minutes.
Roll the chilled dough into balls, about 1 inch in diameter.
Place the dough balls on the prepared baking sheet, spacing them about 2 inches apart.
Make an indentation in the center of each cookie with your thumb.
Fill each indentation with about 1/2 teaspoon of fruit jam.
Bake for 12-15 minutes or until the edges are lightly golden.