Step 1: In a large mixing bowl, cream together the softened unsalted butter and granulated sugar on medium speed until light and fluffy, about 2-3 minutes.
Step 2: Beat in the egg yolk and vanilla extract until well combined, scraping down the sides of the bowl as needed.
Step 3: In a separate small bowl, whisk together the all-purpose flour and salt. Gradually add the dry ingredients to the wet, mixing on low speed until just combined. Do not overmix.
Step 4: Preheat your oven to 375°F (190°C). Line baking sheets with parchment paper or silicone baking mats.
Step 5: Roll the dough into 1-inch balls and place them about 2 inches apart on the prepared baking sheets.
Step 6: Using your thumb or the back of a small round measuring spoon, gently press an indentation into the center of each cookie dough ball.
Step 7: Fill each indentation with about ½ teaspoon of your chosen jam.
Step 8: Bake for 10-12 minutes, or until the edges are lightly golden and the cookies are set.
Step 9: Remove from the oven and let cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely. The jam will be very hot.