Place the two types of chocolate in a heatproof bowl set over a saucepan of simmering water (don't let the bowl touch the water) and stir until melted and smooth. Cool slightly, then beat in egg yolks (this will be extremely thick and curdled looking, but don't worry as the added cream will smooth it out again). Set aside.
Soak gelatine in cold water for 5 minutes to soften. Place 200ml (6.7 fl oz) of the cream in a small saucepan over low heat and bring to a simmer. Squeeze excess water from gelatine and stir into cream until melted and combined. Set aside to cool a little, then beat the cream into the chocolate mixture.
Whisk remaining 700ml (23.6 fl oz) cream to soft peaks in a bowl and set aside. Place egg whites in a stand mixer and whisk to soft peaks. Add sugar, 1 tablespoon at a time, and beat until sugar is dissolved (don't over-beat or mixture will be too difficult to fold in).
Fold whipped cream into chocolate mixture. Then, in 2 batches, fold in egg whites (given the volume of mixture, you may need to separate it out into two bowls, as I did).