So here's the deal. Get two bars of 90% Lindt chocolate. You can use any dark Lindt bars, but the higher the cocao solids content the better, although the 99% is not a good choice it turns out. If you use unsweetened bakers chocolate (or Lindt 99%) add three teaspoons of sugar to the gelatin water mixture in that step. You can use any dark chocolate, but the quality of the mousse is a function of the quality of the chocolate and I really wouldn't bother with cheap or nasty chocolate here, especially if the first listed ingredient is "sugar". You may as well just get some instant pudding if you're gonna do that.
You'll need 4 eggs and a packet of gelatin. It goes a bit quicker if the eggs aren't cold, and doesn't hurt to take them out of the frig for a while beforehand. Nor does it hurt to crack and separate the eggs into separate bowls at the beginning so the have a chance to warm up a bit.
Stick a pyrex measuring cup in a pot of water and put it on the stove on low heat, I use a "3" setting (out of 10). It'll take a while but you CAN'T get chocolate too hot.
While it's melting, and it could take a while, seperate the eggs, yolks in one bowl, whites in another. With a whisk, beat the yolks until they're light colored, thick and foamy. Wask the wisk as any trace of yolk will screw up the whites, then whisk the whites until they are stuff. This can require a hefty amount of arm work, so men: use the arm you don't masturbate with as you'll walk lopsided. Women, as usual resort to electronics and use an electric mixer.
In a small bowl put roughly 1/3 cup of boiling water. Add a packet of geletin to it, stir. (optionally add a couple of tablespoons of double strength coffee, or brandy)
When the chocolate has melted, take it out of the water bath and let it sit so it cools down a bit, not too much but you don't want it too hot that it cooks the yolks you're about to fold in, but not too cool that it's no longer liquid.
Now, using a spoon and not a whisk, pour in 1/3 of the yolk mixture, stir, then add another 1/3, stir, then the last of it, stir. Add the water/geletin, whisk until it's all mixed in. You have to do it this way, really. It'll screw up if you don't and make a wretched mess.
Whisk the whites again to make sure they're good and firm, and using a spatula and not a whisk, add 1/3 of the whites to the chocolate mixture, stirring with, and this bears repeating, a spatula or wooden spoon. Add another 1/3, stir, then the final 1/3, stir. You want to stir it the least amount possible, but you do want it all mixed up.
Pour into small bowls and stick it in the fridge for a minimum of 2 hours. 4 is better.
OMFG! This is the best mousse I've ever had and puts anything in a restaurant to shame.
Optionally, when you do the water geletin thing, add 3 more tablespons of water to it and 2 tablespoons of instant coffee. I loathe instant, but it works fine here. Alternatively, just use just-brewed very strong (I mean at least double strength) coffee instead of water. Or any liquor or liqueur.
Also, using a kitchen fine grater and putting in some clementine zest really makes it too. You can top it with whipped cream by beating some cream, optionally adding 2 teaspoons cocoa power and 1 teaspoon sugar. Instead of cocao power you could add tangerine zest to the whipped cream, or peppermint extract to the unwhipped cream.
The shit is seriously righteous.