Buy a slab of dark chocolate. Go for a chocolate with a high cocoa content (75% cocoa solids and above). I used a Swiss Lindt chocolate with over 85% cocoa solids. Melt it very slowly in a double boiler (ie a bain-marie) with about four tablespoons of water, until it's smooth and shiny. Make sure the chocolate remains warm, rather than hot. Remove it from the heat.
Next add two tablespoons of unsalted butter and a tablespoon of crème fraiche. Mix them in very slowly. Now it's time for the eggs. Take hold of three eggs (kept at room temperature), and separate the yolks from the whites. Add the three egg yolks, one by one to the chocolate mixture. Stir them in very slowly.
In a separate bowl, whisk up the remaining three egg whites. Make sure that the bowl is clean, and there is no trace of egg yolk, otherwise the whites won't get stiff. Whisk them until they are form stiff peaks. Add a pinch of salt, and a tablespoon or so of fructose or white sugar. This will give the egg whites a lovely gloss. Now for the fun bit.
Add a dollop of the stiff egg whites to the chocolate mixture and stir it in very, very gently with a metal spoon. You need to hold your spoon as if it was a feather. What you don't want to do (as I did the first time I attempted this) is to stir it briskly. The lighter your touch, the lighter your mousse. It's a fine art. Slowly stir in the remaining egg white.
Divide the mixture into ramekin dishes, and place them in your 'fridge for at least three hours. If you're in a cheffy mood, you can pipe the mousse into the ramekins in arty swirls, as shown in the photograph. Decorate with shavings of white and dark chocolate. Eat.