Distribute half of the batter between both cups.
Place half of the peanut butter in each cup.
Cover the peanut butter up with the rest of the batter.
Cook the mug cakes in the microwave for 1.5-2 minutes. Add 30 seconds to that if it was in the fridge previously. Eat it straight away! Since it's cooked in the microwave, it becomes less yummy as it cools down.