Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper or silicone baking mats.

In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy using an electric mixer or stand mixer.

Add the egg yolks (reserve the egg whites for later) and vanilla extract to the butter-sugar mixture, and beat until well combined.

Gradually add the flour and salt to the wet ingredients, mixing until a smooth dough forms.

In a separate small bowl, lightly beat the reserved egg whites with a fork.

Roll the cookie dough into small balls, about 1 inch in diameter. Place them on the prepared baking sheets, leaving some space between each cookie.

Use your thumb or the back of a small spoon to make an indentation in the center of each cookie.

Brush the tops of the cookies with the beaten egg whites. This step helps create a shiny finish on the baked cookies.

Fill each indentation with your choice of filling, such as jam, chocolate ganache, caramel sauce, or any other desired filling.

Bake the cookies in the preheated oven for 12 to 15 minutes, or until the edges are lightly golden.

Remove the cookies from the oven and let them cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.

Once cooled, enjoy your delicious thumbprint cookies with a glass of milk or cup of coffee! Store any leftovers in an airtight container at room temperature for up to a week.