1. Preheat the oven to 375 degrees F (190 degrees C).
2. In a medium bowl, cream together the butter, white sugar and egg yolks. Mix in flour a little bit at a time until a soft dough forms. Roll dough into 1 inch balls. If dough is too soft, refrigerate for 15 to 20 minutes. Place balls 2 inches apart onto ungreased cookie sheets. Use your finger or an instrument of similar size to make a well in the center of each cookie. Fill the hole with 1/2 teaspoon of jam.
3. Bake for 8 to 10 minutes in the preheated oven, until golden brown on the bottom. Remove from cookie sheets to cool on wire racks.