First of all place the broken-up chocolate and 4 fl oz (120 ml) warm water in a large heatproof bowl, which should be sitting over a saucepan of barely simmering water, making sure the bowl doesn't touch the water.Then, keeping the heat at its lowest, allow the chocolate to melt slowly – it should take about 6 minutes. Now remove it from the heat and give it a good stir until it's smooth and glossy, then let the chocolate cool for 2-3 minutes before stirring in the egg yolks. Then give it another good mix with a wooden spoon. Next, in a clean bowl, whisk the egg whites to the soft-peak stage, then whisk in the sugar, about a third at a time, then whisk again until the whites are glossy. You can watch how to whisk egg whites by clicking on the Cookery School Video on the rightNow, using a metal spoon, fold a tablespoon of the egg whites into the chocolate mixture to loosen it, then carefully fold in the rest. You need to have patience here – it needs gentle folding and cutting movements so that you retain all the precious air, which makes the mousse light. Next divide the mousse between the ramekins or glasses and chill for at least 2 hours, covered with clingfilm. I think it's also good to serve the mousse with a blob of softly whipped cream on top.Note: this recipe contains raw eggs.