Sift together the flour and salt
Place butter in a large bowl and beat with an electric or stand mixer on high speed for 5 minutes or until pale yellow and fluffy. Add the sugar and beat on medium-high speed until mixture is very pale and very light, about another 2 to 3 minutes. Scrape down sides of bowl as necessary. Add the yolks and vanilla. Beat for 30 seconds or until incorporated. Add the flour mixture and beat, slowly at first, for about 20 seconds, increase speed to medium until mixture forms a rough dough.
Line cookie sheets with parchment paper or silicone mats. Shape rounded tablespoons of dough into a ball using your palms and make an indentation in the middle of the dough with your thumb. Place onto baking sheet about 2 inches apart. Cover with plastic wrap, and refrigerate dough for at least 2 hours.
Heat oven to 350 degrees.
Fill each indentation with ¼ teaspoon jam. Bake for 10 to 12 minutes. The tops should take on a slight toasted color but the bottoms should be browned. Let cookies sit on a cooling rack for 1 hour to set.