Preheat oven to 350°. In a small bowl, combine the first 6 ingredients until smooth; set aside. , In a large saucepan over medium heat, melt butter and chocolate. Remove from the heat and let cool. Stir in eggs and vanilla. Add sugar, flour, baking powder and nuts, stirring until blended. , Spread batter in a 13x9-in. baking pan coated with cooking spray. Spread filling over batter. For topping, in small bowl, combine chocolate chips and nuts; sprinkle over filling. , Bake until almost set, about 28 minutes. Sprinkle with marshmallows; bake 2 minutes longer. , For frosting, in a large saucepan, heat butter, milk, cream cheese and chocolate until melted, stirring until smooth. Remove from heat; stir in confectioners' sugar and vanilla. Immediately drizzle over marshmallows. Chill well; cut into bars.