Preheat oven to 3500F. Grease and flour 13x9-inch pan. In saucepan over low heat, melt 1/2 cup butter and 1 ounce chocolate (if using). Add remaining bar ingredients and mix well. Spread in prepared pan.
Make Filling
In small bowl, combine 6 ounces cream cheese with the next 5 ingredients. Blend until fluffy. Stir in nuts and spread over chocolate mixture. Sprinkle chocolate chips over top. Bake until toothpick comes out clean. Sprinkle marshmallows over cake and bake 2 minutes longer.
Make Frosting
Over low flame in large saucepan, melt 1/4 cup butter, 1 ounce chocolate, remaining 2 ounces cream cheese, and milk. Stir in powdered sugar and vanilla until smooth.
Immediately pour over marshmallows and swirl together. Sprinkle with 1/4 cup chopped pecans, if desired. Store in refrigerator. Cut into squares.