Preheat oven to 350. Prepare chocolate cake mix according to the box. Mix marshmallows, brown sugar, cocoa, and water in a bowl. Pour into a
greased 9x13 pan. Spoon cake mix on top of marshmallow mixture, but don't mix them. Bake for 45 minutes. Then, after the cake cools, flip over onto a cookie sheet. Be careful--the marshmallow mixture might run. No need to frost.