Preheat oven to 350°. In a small saucepan, melt 1 cup butter; stir in 1/2 cup cocoa until smooth. Remove from heat. In a large bowl, beat eggs, sugar and vanilla until blended. Combine flour and salt; gradually add to egg mixture. Beat in cocoa mixture., Transfer to a greased 15x10x1-in. baking pan. Bake until a toothpick inserted in the center comes out clean, 18-22 minutes. Cool on a wire rack for 3-4 minutes., Meanwhile, in a microwave, heat peanut butter, uncovered, on high until softened, about 30 seconds. Stir and spread over warm brownies. Refrigerate until peanut butter is set, about 45 minutes., In a heavy saucepan, combine milk, marshmallows, remaining cocoa and remaining butter. Cook and stir over medium-low heat until butter and marshmallows are melted and mixture is smooth. Remove from heat. Gradually stir in confectioners' sugar until smooth. Spread over peanut butter layer. Refrigerate at least 30 minutes. Cut into squares.