Butter a 9-inch square pan generously. Combine butter or margarine and sugar in a small heavy saucepan. Cook over moderate heat, stirring constantly until candy thermometer registers 310 degrees. Candy should be caramel color. A small amount of syrup should form a hard ball when dropped into cold water and should be brittle when removed from water. Pour candy quickly into buttered pan. Let cool to harden. Melt chocolate pieces in top of double boiler over hot water (or in microwave). Spread half of melted chocolate over surface of hardened candy. Let cool until chocolate is hardened, using a pancake turner, turn candy over and spread remaining melted chocolate. Cool until hardened and break into pieces. To develop flavor, store in a tightly covered tin for at least 2 days. Yield: 1 pound. For extra flavor, crushed almonds may be sprinkled on candy.