In a heavy saucepan, combine 3 cups of sugar, milk, and butter. Let it come to a boil on medium heat. Put 1/2 cup of sugar in a iron skillet. Heat it until melted and it turns dark brown. Add 1/2 cup hot water and cook until the lumps are melted. Empty the skillet into the saucepan with the boiling filling.
Cook until you reach the soft ball stage. Remove from heat. Add baking soda and mix well. Beat until mixture is thick enough to spread on tops and sides of 3 cake layers.
