Combine 3 cups sugar and milk in a large sauce pan; cook over low heat, stirring frequently, until sugar is dissolved. Remove from heat, and set aside.
Place remaining 1 1/2 cups sugar in a 10 - 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 milk mixture in sauce pan. Cook over medium heat, stirring constantly, until the mixture reaches soft ball stage (240°).
Remove from heat, and beat mixture at medium speed of electric mixer about 5 minutes or until thick enough to spread. Spread immediately on cooled cake.
Oxmoor House Homestyle Recipes