Melt the chocolate and butter in a large mixing bowl placed over a pan of barely simmering water, making sure the surface of the water does not touch the bowl. Remove from the heat, add the cocoa powder, 50 g (2 oz) of the sugar, the egg yolks, vanilla extract and cream and beat the mixture to a smooth paste.
Whisk the egg whites in a clean bowl until peaking and gradually whisk in the remaining sugar. Use a large metal spoon to fold a quarter of the meringue into the chocolate mixture to lighten it, then fold in the remainder.
Spoon the mixture into 4 small ramekin dishes or similar-sized ovenproof dishes. Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 8–10 minutes or until a very thin crust has formed over the surface. Dust with icing sugar and serve immediately.