Cook in the microwave on high for 1 minute & 10 seconds (see notes below).
Notes
Egg white: you can swap the egg white for 2 tablespoons of any milk (almond milk, coconut milk, oat milk, soy milk, etc.) but egg white definitely gives the best texture.
Cooking time: This will vary depending on the wattage of your microwave so test at 1 minute first, then increase in 10 second increments until you've found your perfect cook time.
More than one: The cooking time will also vary if you cook more than one mug cake at a time.
Smaller Cakes: Batter can be divided into ramekins for smaller cakes but these will take less time to cook.