Sometimes, you just want a treat. When that feeling hits, why not make a treat in a coffee mug? That way, you only eat one serving and let's face it: It is much faster to whip something up in a single serving, than it is to make an entire batch of brownies.
I know from experience that it is possible to eat an entire pan of brownies. Don't do it! Make one of these desserts in a mug instead.