Preheat your oven to 350°F and line two baking sheets with parchment paper.
In a large bowl, cream together the butter and sugar until light and fluffy, about 3 minutes. Beat in the egg yolks and vanilla extract.
In a separate bowl, whisk together the flour and salt. Gradually mix the dry ingredients into the butter mixture until just combined.
Roll the dough into 1-inch balls and place them about 2 inches apart on the prepared baking sheets.
Using your thumb or the back of a small measuring spoon, make an indentation in the center of each cookie. Fill each indentation with about 1/2 teaspoon of jam.
Bake for 12-14 minutes, until the edges are lightly golden. The cookies will look soft but will firm up as they cool.
Let cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. Dust with powdered sugar if desired before serving.