Combine all ingredients in a 2-quart saucepan. Cook over medium heat, stirring constantly, until butter melts and sugar dissolves. Cover and continue to cook over medium heat 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, without stirring, until mixture reaches hard ball stage (260°).
Remove from heat, and beat with a wooden spoon until mixture is creamy and begins to thicken. Working rapidly, spread mixture in a buttered 8- inch square pan. Mark top of warm candy into 1-inch squares, using a sharp knife. Cool completely before cutting into squares. Wrap each piece tightly in waxed paper or plastic wrap.