Combine 2 cups sugar and whipping cream in a medium saucepan; cook over low heat, stirring frequently, until sugar dissolves. Remove from heat, and set aside.
Place remaining 1 cup sugar in a 9-inch cast-iron skillet; cook over medium heat, stirring constantly, until sugar dissolves and becomes a golden syrup. Add butter, stirring well.
Gradually pour syrup mixture into reserved cream mixture in saucepan. Cook over medium heat, stirring constantly, until mixture reaches soft ball stage (240°).
Remove from heat, and beat mixture at medium speed of electric mixer 5 minutes or until thick enough to spread. Spread immediately on cooled cake.