Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper.
In a large bowl, cream together the softened butter and granulated sugar until the mixture is light and fluffy, about 3 minutes.
Add the egg yolks one at a time, beating well after each addition, then stir in the vanilla extract.
Gradually add the flour and salt to the butter mixture, mixing on low speed until a soft dough forms; avoid overmixing to keep the cookies tender.
Roll the dough into 1-inch balls and place them on the prepared baking sheets, spacing them about 2 inches apart.
Use your thumb or the back of a small measuring spoon to press a deep indentation into the center of each dough ball.
Fill each indentation with approximately 1/2 teaspoon of the thick sorrel jam.
Bake for 12 to 15 minutes, or until the edges of the cookies are lightly golden and the centers are set.
Remove from the oven and allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.