Cream butter and sugar. Blend in vanilla, add flour and mix well. Refrigerate several hours or overnight. Form dough into small balls - about the size of a nickel. Roll in sesame seeds and place on cookie sheets, 3" apart. Make small indentation in center of each cookie and fill with preserves (do not mound or the preserves will bubble onto cookies during baking). Bake at 375° for 10-12 minutes. Makes about six dozen.