I've tried quite a few of these mug cake recipes and this is by far the easiest, not to mention the best tasting and the best texture. Many of them turn out very spongey, but this one is perfect.
Here's what you need:
1/4 cup cocoa powder
1/4 cup powdered sugar
And....that's it! You can add a splash of vanilla and any extra bits if you desire. We often drop in a handful of chocolate chips and/or marshmallows to kick it up a notch.
Mix them all together (the egg is beneath the sugar and cocoa in this pic).
Pop in the microwave for 30-50 seconds. My daughter likes her very dry so we go for 50 seconds for her. For a moister cake, go for 30-40.
You end up with a crazy looking lump of a cake, but it's delicious!! You can top with powdered sugar, ice cream, or whipped cream, but this cake is plenty sweet enough to not need anything extra.
(I don't remember where I found this recipe, it's been quite awhile, but similar recipes can be found at Babble, and Chocolate Chocolate and More