In a large bowl, whisk together the flour, sugar and salt.  Then, add in the butter, blending the mixture until it resembles coarse meal (consider using your hands, it is much easier!).  Stir in the egg yolks, blending the mixture until it forms a dough.  Chill the dough, wrapped in plastic wrap, for at least two hours, or overnight.

Preheat the oven to 350 F.  Let the dough soften for a few minutes, then roll little balls out of the dough.  Use a cookie scoop like this one for evenly shaped balls. Arrange them 2 inches apart on a lightly greased cookie sheet.  Then, press your thumb in the center to create the well for the jam.  If the dough cracks, just re-roll it, or use a little water to press it back together.

Fill each little well with a bit of jam and bake the cookies for 12-15 minutes or until the cookie is lightly golden.

Let the cookies cool for a couple of minutes on the sheet, then transfer them to racks and let them cool completely.