This was the best chocolate mousse I have ever made: richly frothy in texture, and without the graininess that has marred my efforts in the past. I think the key was that the chocolate mixture was properly cool before I added the egg yolks.
The ingredients produced enough mousse to fill seven ramekins.
150g dark chocolate (I used Green & Black's cook's chocolate)
25g butter, cut into small cubes
6 eggs, separated
Break the chocolate into its squares, and melt it in a bowl held above a pan of simmering water. (I have a Pyrex bowl that rests on the edges of a saucepan.) Stir the chocolate to encourage melting, and remove it from the heat as soon as, or slightly before, all the lumps have disappeared. Drop in the butter.
Beat the egg whites, just on their own and with no salt or acid (see this post), until they form soft peaks. Take about a quarter of the white and beat it into the chocolate and butter. The mixture should retain the texture and consistency of thick chocolate sauce. Now stir in the yolks.
Pour the chocolate mixture over the egg whites. Fold it in, using a lifting and turning motion with the spoon until amalgamated. Transfer to a dish, or to individual ramekins, and refrigerate for at least three hours.