Step 4: Experiment!
This cake is tricky mostly because of people's microwaves all being different. I experimented a lot with the amounts of different ingredients.
I think if it's something you're going to make quite a lot in the future, it's worth fussing around with! Try just a bit less time in the microwave (you can always put it back in for more). If that doesn't help, try a little less egg, or a little less flour.
Keep notes on what you're trying each time, and eventually you'll find the cake that's perfect for both you and your microwave!