Hervé This' Chocolate Mousse

Directions

1. 1
   Simply pour water into a saucepan (which will be improved from the gastronomic point of view if it is flavored with orange juice, for example, or cassis puree). Then, over medium-low heat, whisk in the chocolate. The result is a homogenous sauce.
2. 2
   Put the saucepan in a bowl partly filled with ice cubes (or pour into another bowl over the ice-it will chill faster), then whisk the chocolate sauce, either manually with a whisk or with an electric mixer (if using an electric mixer, watch closely-it will thicken faster). Whisking creates large air bubbles in the sauce, which steadily thickens. After a while strands of chocolate form inside the loops of the whisk. Pour or spoon immediately into ramekins, small bowls or jars and let set.
3. 3
   Note: Three things can go wrong. Here's how to fix them. If your chocolate doesn't contain enough fat, melt the mixture again, add some chocolate, and then whisk it again. If the mousse is not light enough, melt the mixture again, add some water, and whisk it once more. If you whisk it too much, so that it becomes grainy, this means that the foam has turned into an emulsion. In that case simply melt the mixture and whisk it again, adding nothing.
4. 4
   Serve immediately, or refrigerate. Top with whipped cream if desired.