Combine all ingredients in a small bowl and stir thoroughly to combine and form a batter.
Decant the batter into a microwave safe mug and microwave for 1 minute, then assess. If it remains undercooked, cook again for 20 seconds or until there is no spongy surface to the mug cake. All microwaves vary greatly in speed and power so keep an eye on it.
Allow to cool for 2-3 minutes before eating to allow the cake to finish cooking. Results can be dense or mushy if you don't give it this extra few minutes (it's worth the wait!)
Notes
Canned pumpkin puree isn't easily accessible in Australia, so I use steamed Japanese pumpkin which I mashed with a fork.
I have tested this recipe with Libby's canned pumpkin and it works nicely. However, I found the canned version to be a lot more savoury than fresh, so I recommend using a little extra sugar to compensate.
The tapioca flour is optional, but gives the cake a tiny bit of chew, akin to a cake with eggs. You can omit it if you'd like - the cake will just be fluffy without any elasticity to the crumb. You might need to add a teaspoon less milk.
A gritty, stale rice flour will result in an unpleasant mug cake. Make sure yours is fresh and super finely ground.
Add spices to your tastes and preferences. You can use pre-bought pumpkin spice, if you would like. Again, this isn't easily accessible in Australia but add as per your tastes.
To keep this pumpkin mug cake vegan, use a plant based milk. To keep it nut free, use a nut free milk.
There are notes on making the mug cake in an oven in the body of the post. I recommend adding 1-2 tsp less oil to the oven baked version as it seems to need less.