In a medium mixing bowl cream butter and sugar,
throughly beat in yolks and vanilla with a wooden spoon,gradually add flour until blended. Chill. Shape dough into 1 inch balls .Place balls 1 1/2 inches apart on ungreased cookie sheets.Using a lightly floured thimble,press a small hole about 1/4 inch deep in the top of each cookie. Fill each hole with jam.Bake in preheated oven until browned on bottom but not on top about, 25 mins. Remove to wire racks to cool. Store in tightly covered tin box.