In a bowl, or right in the mug, whisk flour, sugar, salt, baking powder, and soda. Add egg and combine until just incorporated with dry ingredients. Mix in vanilla, oil, milk and mashed banana.
Pop into the microwave for up to 3 minutes, with no fear to stop and check after 90 seconds for doneness.
Like all magical things that come out of the microwave, the inside of this will be hotter than the surface of the sun when you first remove it from the microwave. Give it a few minutes.