Use a ceramic mug that is microwave-safe and large enough to allow your cake to rise. Do not fill batter to the brim as the cake will rise significantly during cooking, which will cause your batter to overflow. I find that a standard 8oz coffee mug is just about the right size for this eggless mug cake. If you do not have the right mug, you can use a 8-12oz small bowl or ramekin instead. You might have to reduce the cooking time slightly, by about 10 seconds.