3generous tablespoons very strong hot black coffee
60g (2oz) caster sugar
1 large saucepan to serve as a bain-marie for the bowls
1 2 litre (3½ pint) china bowl
Put the bitter chocolate, broken into pieces, into a large bowl together with the hot black coffee and the unsweetened cocoa powder. Place the bowl over the saucepan of water and heat gently. When the chocolate has melted stir the mixture well until it is thoroughly blended. Take the bowl out of the bain-marie.
Clean the second bowl carefully, rubbing the interior with half a lemon. Rinse under cold water and dry carefully. Put in the egg whites which must on no account be chilled, add a pinch of salt and whisk until they start to form a light snow. At this point add the sugar and continue to whisk until the egg whites are dry and very firm.
Still using the whisk incorporate a quarter of the egg whites into the warm chocolate mixture. Now fold in the rest of the egg whites using a spatula, not the whisk, so that the egg whites do not fall. Transfer this mousse to a china bowl and place in the refrigerator for an hour to chill.