Place sugar, Dr Pepper, grated chocolate and corn syrup in heavy sauce pan. Cook very slowly, stirring constantly until sugar and chocolate is thoroughly dissolved. Continue cooking on low medium heat until temperature of 236? or soft ball stage is reached. Remove from heat and cool at room temperature to lukewarm, 110?. Add butter and vanilla. Beat until the candy loses its shiny look; add nuts, if desired. Pour into slightly buttered pans. When cold, cut into squares.