Bake brownie mix according to package directions. While the brownie mix is still hot sprinkle with mini marshmallows. Combine butter, baking cocoa and milk in medium saucepan. Bring to a boil over medium heat, stirring constantly with a wire whisk and boil one minute. Remove from heat and stir in powdered sugar. Immediately pour chocolate mixture over marshmallows. Let cake cool completely then cut into squares.