I prepared my "Blue Monday Cake" (recipe here from a 2012 blog post). I didn't have any white flour, so I used a mix of spelt and whole wheat - to Terry's chagrin! I also added an egg, as most of the mug cakes recipes online seemed to have eggs in them.
I filled the containers about 1/3 full.
I started out by microwaving them for about 20-30 seconds on high power - give or take - depending on the size of the container. I then switched to 70% power for 40-60 seconds. If, after testing with a toothpick, the batter was still runny, I added another 10 seconds on high. I baked the bowls individually, on a paper towel - in case there was spillage. (There wasn't any!)
And, even though I didn't grease the pottery bowls, they cleaned up very easily!