Preheat oven to 350 degrees F. Line two large baking sheets with parchment paper.

In a large bowl, cream together the butter and sugar until light and fluffy.

Add egg yolks and vanilla. Blend well.

Slowly mix in flour and salt until fully combined. The dough may look crumbly at first but keep mixing and it will begin to hold together.

Use a medium cookie scoop to portion out 1 1/2 tablespoons of dough and roll into a ball. Roll the dough in sugar then place 2 inches apart on the parchment-lined baking sheet.

Use your thumb (or the back of a small cookie scoop) to create an indention in the middle of each ball of dough.

Fill the indention with half a teaspoon of jam.

Bake at 350 degrees F for 12-14 minutes, until the cookies are light brown.