Cream butter and sugar. Add egg yolks and vanila and beat until light and fluffy. Stir in flour and, if necessary, chill until firm enough to handle. With floured hands, roll in 1" balls. Put 1 1/2" apart on cookie sheets. Using a lightly floured thumb, make a small indentation in the center of each cookie. Fill with jam. Bake in preheated 325 degree oven for about 25 minutes. Cool on racks. Makes about 4 doz. (Source: Marceline Hill)