In a large bowl, cream together the softened butter and granulated sugar using an electric mixer until light and fluffy. This usually takes about 2-3 minutes on medium speed.

Beat in the egg yolk and vanilla extract until well combined. Scrape down the sides of the bowl as needed.

In a separate bowl, whisk together the flour and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined and a soft dough forms. Be careful not to overmix.

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

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

Using your thumb or the back of a small spoon, gently press an indentation into the center of each cookie ball, creating a well for the jam. Be careful not to press all the way through the dough.

Fill each indentation with about 1/2 teaspoon of your chosen jam or preserves. Don't overfill, as the jam will spread slightly during baking.

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

Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.