Preheat oven to 180 degrees C. Grease a 20x30cm baking dish.
In a large bowl, combine the cake mix, milk, butter and egg; mix until well blended. Batter will be thick. Stir in the chocolate chips. Stir in the walnuts if desired. Spread the batter evenly into the prepared pan.
Bake for 25 to 30 minutes in the preheated oven until brownies spring back to the touch. Cool before cutting into squares.