In a microwave-safe coffee mug (minimum size 1 cup/250ml), whisk together peanut butter, maple syrup, and vanilla until smooth.
Whisk in almond milk, at room temperature, baking powder, and ultra-fine oat flour (or flour of choice) until it forms a light batter with no lumps.
Stir in vegan dark chocolate chips if desired, or keep the batter plain.
Sprinkle few extra chocolate chips on top if desired.
Microwave on high power (800W) for about 70 seconds or stop the microwave 10 seconds after you see the cake rising to its maximum height. Remove from the microwave carefully. It is hot! Check the texture. It should be moist and soft, and the top should deflate. If it is not cooked to your liking after that time, return to the microwave and cook again by 10-second bursts until done - no more than 90 seconds in total. Note that the longer you microwave the mug cake, the denser it will be.
Serve immediately with a dollop of vegan ice cream or plain.
Preheat oven to 350°F (180°C)
Grease an oven-proof ramekin with oil, add the cake batter and bake for 12-15 minutes until cooked through.