It fits perfectly in here, and also in Cakes, etc. I will copy it in there. However, I will also print out the directions, since blogs (like the one you linked to) have a way of disappearing after a few years.
Ingredients:
4 Tablespoons cake flour
4 Tablespoons sugar
2 Tablespoons cocoa
1 Egg
3 Tablespoons milk
3 Tablespoons oil
1 Mug
Instructions:
Mix flour, sugar and cocoa
Spoon in 1 egg
Pour in milk and oil, and mix well
Put in microwave for 3 minutes on maximum power (1000watt)
Wait until it stops rising and sets in the mug
Tip contents out of mug onto saucer and enjoy!
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My only suggestions are that you probably should use a microwave safe cup,
like the ones sold here (the first two mugs on almost every page) 
, and also use a food release spray to coat the mug before you put the other ingredients in. (You may be able to use spray margarine. I may report back on this shortly.
This would also be a great idea for a children's party at which there aren't a lot of guests. Little kids would enjoy making their own cake.