Preheat broiler. Sprinkle baked brownies with cereal and marshmallows; broil 5-6 in. from heat until marshmallows are golden brown, 30-45 seconds. Immediately sprinkle with chopped chocolate. Cover with foil and let stand until chocolate begins to melt, 5 minutes. Remove foil and cool completely in pan on a wire rack. Cut into bars.

Preheat oven to 350°. In a large bowl, cream butter and sugar until light and fluffy. Beat in eggs and vanilla. In a bowl, mix flour, cocoa and salt; gradually beat into creamed mixture. Spread into a greased 13×9-in. baking pan. Bake 25-30 minutes or until a toothpick inserted in center comes out with moist crumbs (do not overbake).