Cream butter and sugar and add egg. Beat well, add flour and salt. Mix dough by hand until soft. Refrigerate for 2 hours. Roll on floured board to 1/4 inch thick. Cut into rounds. Cut center out (using the top of the vanilla extract cap) of 1/2 of rounds. This is the top of the cookies. Bake at 375 degrees for 7-10 minutes or very lightly brown. Cool and fill with favorite preserves. Sprinkle with confectioners sugar.