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

In a large mixing bowl, using an electric mixer (or by hand with a whisk), cream together the softened butter and granulated sugar until the mixture is light, fluffy, and pale yellow. This usually takes about 2-3 minutes.

Beat in the egg yolks one at a time, mixing well after each addition. Then stir in the vanilla extract. Scrape down the sides of the bowl to ensure everything is well combined.

In a separate medium bowl, whisk together the flour and salt. Combine these thoroughly before adding them to the wet ingredients.

With the mixer on low speed, gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix the dough, as this can lead to tough cookies.

Roll the dough into 1-inch balls. Place the dough balls about 1-2 inches apart on your prepared baking sheets.

Dip your thumb or the back of a small measuring spoon into flour to prevent sticking, and then gently press an indentation into the center of each cookie dough ball.

Carefully spoon a small amount of your chosen jam into the center of each thumbprint. Don’t overfill; a generous teaspoon per cookie is usually perfect.

Bake for 12-15 minutes, or until the edges of the cookies are lightly golden. The jam should be bubbly and set.

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