Preheat oven to 180 degrees C. Prepare brownie mix according to packet directions. Spread into a greased 20x30cm pan.
In a medium bowl, break biscuits into pieces and toss with miniature marshmallows and milk chocolate. Set mixture aside.
Bake brownies for 15 minutes in the preheated oven. Remove and sprinkle the chocolate/marshmallow/biscuit mix evenly over the top. Bake for an additional 15 to 20 minutes, or until a skewer inserted in the centre comes out clean. Allow brownies to cool before cutting into squares.