Melt bars and 1/2 C. butter in saucepan over very low heat. Beat remaining 1/2 C. butter and sugar in med. size bowl until fluffy. Add eggs, one at a time, beat well. Add flour and baking soda alternately with buttermilk, stir until smooth. Add melted candy, mixing well, stir in vanilla and nuts. Bake in moderate oven 350 for 1 hour and 20 min., or until top springs back when lightly touched with fingertip.(top will be quite dark). Cool in pan on wire rack 10 min. Remove from pan, cool completely. Frost if desired.