Ultra-Rich Brownie Bars

Name: Ultra-Rich Brownie Bars
Yield: Makes about 2½ dozen.
Ingredients: 4 cups confectioners’ sugar, sifted; 2 oz. cream cheese, softened to room temperature; ⅛ tsp. salt; 1 large egg, lightly beaten; ¼ cup granulated sugar; ¼ cup confectioners’ sugar, sifted; ⅓ cup unsalted butter, softened to room temperature; ⅓ cup milk, whole or 2 percent; 2 oz. unsweetened chocolate, coarsely chopped; 2 cups mini marshmallows; 4 large eggs, lightly beaten; 1 tsp. pure vanilla extract; 1¼ cups unbleached, all-purpose flour; 1 tsp. baking powder; 1 tsp. salt; 1 cup chopped walnuts or pecans, lightly toasted; 1 cup chopped walnuts or pecans, lightly toasted; 12 oz. cream cheese, softened to room temperature; 4 oz. unsweetened chocolate, coarsely chopped; ⅔ cup unsalted butter; 2 cups granulated sugar