Spray a microwave-safe mug lightly with oil or butter.

Stir brownie mix, milk, oil, water, and salt in the mug until smooth; fold in chips or nuts.

Microwave on high for 40–60 seconds, checking at 40; cake should be set but slightly gooey.

Let rest 1 minute before eating; top with a spoonful of ice cream if you want to feel fancy.