Choose the mug/jar that the cake will be cooked in. A coffee mug is the ideal size. A smaller mug/jar will work, but it might spill over a bit in the microwave.
In the mug/jar, combine the rice flour, millet flour, sorghum flour, sugar, baking powder, vanilla, and salt. Stir until well mixed.
Add the oil and water to the mug/jar. Stir until evenly mixed.
Microwave on full power for 1 minute 15 seconds. Check the doneness of the cake. If it is undercooked, microwave for up to 30 more seconds.