Here's what you'll need.
3/4 cup (6 ounces) water
8 ounces of your favorite high-quality chocolate
two aluminum mixing bowls — one larger than the other
(don't let this "tools" list scare you — this is going to be easy!)
Put water and the chocolate — broken into pieces like this — into a saucepan.
Stir over medium heat until the chocolate melts.
When it looks like this, you win. Take it off the heat.
Now, chill the melted chocolate in an aluminum bowl nestled in another bowl full of ice.
Then whisk, hard, for several minutes.
Don't give up. Keep whisking.
The reason this works is that you're making an emulsion: As you whisk, microscopic bits of water get suspended in the fat, thickening it and making it seem creamier. (It's the same process for a vinaigrette or mayonnaise.) Then still more air is whipped into it and the cooling chocolate crystallizes around the air bubbles to make a stable foam, aka mousse.
The best thing about it — aside from its dumbfounding simplicity — is that it tastes like pure, unobstructed chocolate. There's no cream or egg to confuse the issue, like in normal mousses. It also happens to be vegan, if you use dark chocolate without any added milk.
Get into the science of it here.