A follow-up from Diane, a friend of the friend who originally sent this to me. (I'm still waiting for a grandchild visit so we can do it together.)
"This definitely works. And, the visual - through the microwave window - is outstanding and will astound any child (and adult!). [See the wonderful photo
My microwave nuked it when it went for 3 minutes. I cut it back to 2:45 and next time will try 2:35.
I didn't add chocolate chips
I used a cocoa that had sugar in it.
There is always gunk in the bottom of the mug that is hard to get out.
I used a small dish to mix the egg and liquids - just made it easier for a 8 year old.
The end result is more science than gourmet cooking - the cake tastes like anything baked too long in a microwave. However, add a scoop of ice cream and it's quite yummy! Don't let it cool - it's hard."