Heat oven to 350°F Grease bottom only of 13 x 9 inch baking pan. Set aside.
Melt 1 cup of butter and 4 oz. unsweetened chocolate in 2 quart saucepan over low heat, stirring occasionally, until smooth. Remove from heat.
Stir in sugar and vanilla. Add eggs, one at a time, mixing well after each addition. Add flour, baking powder and salt; mix just until all ingredients are moistened and brownie mixture is smooth.
Spread brownie mixture into prepared baking pan. Bake for 35 to 40 minutes or until brownies just begin to pull away from sides of pan.
Meanwhile, combine 1/4 cup of butter, cream cheese, 1 oz. chocolate and milk in 2 quart saucepan. Cook over medium heat, stirring occasionally, until melted. Remove from heat. Stir in powdered sugar and vanilla until smooth. Stir in marshmallows and peanuts. Immediately spread over hot brownies. Cool completely; cut into bars. Store refrigerated.