You will need:
1 small egg
1 tablespoon milk
optional: chocolate chips or any other filling
1) Place the flour, sugar and cocoa powder into the mug, mix it around to get rid of any lumps.
2) Add the egg and stir together.
3) Add the milk and oil, then mix thoroughly (yes it looks pretty odd, but it'll turn out fine!)
These are so yummy I really recommend them. It might be a quite dangerous to be only 5 minutes away from delicious cake at any point in the day though...
If you want to you can replace a teaspoon of cocoa powder with coffee powder for a mocha cake, or add dried fruit, chunks of chocolate bars etc. into the cake instead of chocolate chips.
I hope you enjoy the recipe, and that it doesn't cause you to eat way too much cake (I've heard that there IS such a thing, unfortunately...)