Preparation time: 20 minutes, plus 2 hours chilling
165 g (51/2 oz) dark chocolate, at least 70% cocoa solids, finely chopped
25g (1 oz) unsweetened cocoa powder
10 organic or free range egg whites
25 g (1 oz) caster sugar
1 organic or free range egg yolk
Whisking the egg whites:
With an electric beater, whisk the egg whites and sugar for 2–3 minutes, until they form soft peaks.
Adding the eggs to the chocolate:
Stir the egg yolk into the chocolate and cocoa mixture and immediately whisk in a quarter of the egg whites to lighten the mixture.
Melting the chocolate:
Place the chocolate and cocoa powder in a large bowl set over a pan of hot water and leave to melt over a low heat; do not boil the water or the chocolate will become grainy. Stir until smooth, then remove from the heat. Keep warm over the pan of water while you whisk the egg whites.
Fold in the remaining egg whites with a large spatula, ensuring that you do not over mix or the mousse will be heavy. Pour into a glass bowl or individual glasses and leave to set in the fridge for 2 hours or until required.