Whisk together half of the dry ingredients, excepting the chocolate chips, in each mug. Add the wet ingredients and whisk well to combine. Sprinkle chips lightly over the top of the batter and cover the mugs with plastic wrap.
Microwave each mug separately on 50% power. I followed Not Quite Nigella's recommendation and used my microwave's "Melt Chocolate" function3, which took about 90 seconds for a large mug and 60 seconds for a small mug. Check after 60 seconds - the cake should puff up nicely. When it looks like a nice, moist cake, stop. It will cook a bit further due to carryover.
Serve with ice cream4 and milk.
If you don't have vanilla sugar, add 1/2 tsp vanilla extract.
I first tried this with big 16 oz "mugs" that I use for French Onion Soup. Latte mugs (those big, wide mugs you get cafe au lait in at posh coffeebars) would be very nice here as well. If you use standard 10 oz coffee mugs, be careful when splitting between two as it will quite likely boil over.
My microwave is 1100W. If you don't have a Melt function (most microwaves don't), then set the Power Level to 50%.
For extra decadence, top with hot fudge or chocolate syrup.