1. Prepare the Crust:

1. Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.
2. In a large bowl, cream together butter and sugar until light and fluffy. Beat in egg yolks and vanilla.
3. Gradually mix in flour and salt until dough comes together.
4. Roll dough into 1-inch balls and place on prepared baking sheets. Use your thumb to make an indentation in the center of each cookie.
5. Fill each indentation with about 1/2 teaspoon of jam.
6. Bake for 12-15 minutes or until edges are lightly golden. Cool on wire racks.