Line bottom and sides of a 9" cake pan with foil, heavily buttered. Combine butter, sugar and corn syrup in a 10" skillet. Bring to a boil over medium heat, stirring constantly, until mixture begins to turn brown, about 5 or 6 minutes. Add hazelnuts. working quickly, spread mixture evenly into prepared pan. Cool 15 minutes. Remove from pan and foil. Cool thoroughly and break into pieces. Yields about 3/4 pound.