Combine bars and 1 stick butter in saucepan. Cook over low heat, stirring constantly until melted. Set aside.
Cream 2 cups sugar, and 1 stick butter in mixing bowl. Beat in eggs.
Sift flour and baking soda and add to creamed mixture alternately with buttermilk. Stir in candy mixture. Add nuts, if desired, and mix well.
Pour into greased and floured 9x13 pan. Bake at 325 for 1 hr. 10 mins. Combine remaining 2 1/2 cups sugar, evaporated milk, and remaining stick of butter in a saucepan and cook to soft-ball stage, stirring frequently.
Remove from heat and add chocolate chips and marshmallow crème. Cool slightly. Beat until thick and spread over cake. ( I spread frosting while cake is still warm.)