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 extract.
3. Gradually mix in flour and salt until dough comes together. Roll dough into 1-inch balls and place on prepared baking sheets.
4. Press your thumb into the center of each ball to create an indentation. Fill each indentation with about 1/2 teaspoon of jam.
5. Bake for 12-15 minutes, or until edges are lightly golden. Cool on baking sheets for 5 minutes before transferring to a wire rack.