Unless you count water as an ingredient, the only thing this chocolate mousse requires is, well, chocolate. The trick is in what you do with it, and this video from Epicurious shows you how it's done. Spoiler: It may be simple, but it's a good bit of work. You'll need 283g of chocolate — preferably dark chocolate, since it will melt easier and doesn't contain quite as much oil. Add enough water to cover the chocolate. Melt it down over medium heat until it's smooth and easily poured, then transfer to a mixing bowl inside a larger bowl of ice water. That's when the work starts. Whisk feverishly until it starts to thicken up, and then keep whisking until it gets that rich, mousse-like consistency, at which point it's ready to serve.
That's all there is to it. Eat it up fast, it will probably firm up quickly, and you'll also need to pay attention while whisking so the chocolate doesn't seize or lose the consistency you want, but it's easy and you can do it anytime. Assuming you have the chocolate, that is.
How to Make 1-Ingredient Chocolate Mousse [Epicurious (YouTube)]