Preheat your oven to 350°F (175°C) and line your baking sheets with parchment paper
In a mixing bowl, cream together the softened butter and sugar until light and fluffy.
Add the egg yolks and vanilla extract, mixing until well combined.In a separate bowl, whisk together the flour and salt.
Gradually add the dry ingredients to the wet mixture, mixing until a soft dough forms.
Roll the dough into 1-inch balls and place them on the prepared baking sheets.
Make an indentation in the center of each cookie with your thumb (hence the name!).
Fill each indentation with a small spoonful of your favorite jam.
Bake in the preheated oven for 12-15 minutes or until the edges are lightly golden.
Allow the cookies to cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.