1. Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
   
2. Prepare your Mixture

• In a large mixing bowl, use an electric mixer to cream the softened butter and granulated sugar together until light and fluffy, about 2-3 minutes.
• Mix in the vanilla extract and egg yolk until well combined.
• In a separate bowl, whisk together the flour and salt.
• Gradually add the dry ingredients to the butter mixture, mixing until just combined. The dough will be thick.
  

1. Form the Cookies

• Scoop out small portions of dough and roll them into balls, about 1 inch in diameter.
• Place the dough balls onto the prepared baking sheet, spacing them about 2 inches apart.
• Use your thumb or the back of a small spoon to make an indentation in the center of each dough ball.
• Fill each indentation with about 1/2 teaspoon of jam. Be careful not to overfill, as the jam can spill over during baking.
  

1. Bake the Cookies

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

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