170g fine quality dark chocolate, roughly chopped. A minimum of about 60 per cent cocoa solids is fine, but anything more is too rich.
6 free-range egg whites
1/4 teasp lemon juice
40g caster sugar
Gently melt the chocolate in a heatproof bowl placed over a pan of simmering water. Be careful not to let the bottom of the bowl come into contact with the water, or to let the water boil, or the chocolate will seize.
Whisk the egg whites and lemon juice in a large clean bowl until they reach the soft peak stage. The lemon juice helps to stabilise the egg whites.
Add the sugar to the egg whites and continue to whisk until they form firm peaks when you remove the whisk. Be careful not to over whisk or you will beat out all the air and the egg whites will collapse.
Remove the melted chocolate from the heat and quickly whisk in one third of the egg whites.
Using a spatula, fold in the remaining egg whites until you can see no more flecks of white. Again be careful not to overmix or you will knock out some of the air and the final result will not be as light as it could be.
Divide the mousse between four serving glasses and chill for about two or three hours until set.