These amounts are for one mug cake but I usually make two at the same time. If you do, make sure you microwave them separately. I use slightly oversized coffee mugs so this would probably overflow in a small one.
Mix together flour, sugars, cocoa, salt and baking powder in the mug.
Add milk, oil, water, and vanilla and mix until just combined.
Microwave for 2 minutes or until center is done. Time may vary according to mug shape and power.