Put the egg whites in a bowl and whisk with electric beaters until soft peaks. Add in the caster sugar a spoonful at a time, while whisking, until glossy. Put 3 of the egg yolks (you can freeze the remainder – or use them in the aïoli recipe on page 82) into a separate bowl and whisk until pale, then carefully fold in the sugary whites. Stir a few spoonfuls of the egg mixture into the cooled chocolate to incorporate, then gently fold in the remaining egg mixture (try not to knock out all the air). Spoon into 4 glasses, cover and chill for a few hours to set.