Cake: Melt bars and 1/2 cup butter over low heat. Beat other 1/2 cup butter and sugar until creamy. Add eggs, one at a time. Beat. Mix flour and soda; add alternately with buttermilk, beat until smooth. Add candy, vanilla and nuts. Bake at 350 degrees for 1 hour to 1 hour and 20 minutes in 9 inch tube pan.
Topping: Melt bars and butter. Add remaining ingredients and mix.