In a microwave safe mug, whisk together flour, sugar, cocoa powder, baking soda, and salt.
Stir in milk, oil, water, and vanilla extract.
Cook in the microwave for 1 minute 45 seconds, or until the cake is cooked in the middle. Pull out the mug and let it rest for a couple of minutes. (see notes)
Dust with powdered sugar and serve.
Notes
Since microwaves and mug sizes can vary, it might take a few tries to get the baking time just right. The cake should be soft and moist but not wet on top, and it should feel spongy when you press on it.