Chocolate extract is alcohol infused with cacao beans; it's the chocolatey equivalent of vanilla extract. It is awesome. Per David Lebovitz, my baking hero, "when cacao beans are ground up into chocolate, some of the 'top notes' of the beans are lost in the process and adding chocolate extract replaces them."
This stuff is little-known, and I certainly hadn't heard of it before I started baking for a living. Its use is admittedly limited. Unlike vanilla extract, which can be used to complement a variety of other flavors (chocolate / fruit / coffee), chocolate extract can only be used in chocolate-based desserts.
I don't think I've ever seen a recipe actually call for this (though I'm sure they're out there). Once I got a feel for how much to use, I started using it in everything I could. It's very potent stuff (it smells like pure delicious insanity), and just a teaspoon or so is enough to amplify the flavor of a cake or batch of cookies. A general rule of thumb is to use half as much chocolate extract as the amount of vanilla extract called for. When in doubt, use half a teaspoon.
I'm not too proud to make brownies from a box mix once in a while, but when I do, I can't help but throw in a pinch of salt and a dash of chocolate extract, and the flavor is much improved. I haven't tried to fool anyone into thinking they're homemade... but I bet I could.
This is available in specialty groceries, baking supply shops, and of course Amazon. It's a bit pricier than vanilla, but worth it. You can make your own by steeping cacao nibs in vodka; the only thing stopping me is that I've read it can take a year before it's ready to use. (This might be nonsense; I'll experiment at some point and report back.)