Put the chocolate and cream in a heatproof bowl set over a saucepan of gently simmering water (do not let the bowl touch the water) and stir until the chocolate has melted. Leave to cool for 5 minutes, then beat in the egg yolks one at a time.
Whisk the egg whites in a separate clean bowl until stiff, then lightly fold into the chocolate mixture until combined. Spoon the mousse into 4 dessert glasses or cups and chill for 2 hours. Dust with cocoa powder before serving.