The beauty of a mug cake is you can make it quickly and eat it fresh, but yes, you can eat a mug cake the next day. If you need to store your mug cake for later consumption, then let it cool, cover it with plastic wrap or aluminum foil or transfer to an airtight container and store for up to 36 hours at room temperature or up to 5 days in the fridge. Reheat your mug cake in the microwave for 10-15 seconds when ready to eat.