For The Dark Chocolate Mousse
250g/9oz dark chocolate
142ml pot double cream
4 eggs, separated
For The White Chocolate Mousse
250g/9oz white chocolate
284ml pot double cream
For the dark chocolate mousse, break 200g dark chocolate into a bowl, then melt over a pan of barely simmering water. Remove from heat and cool for 5 mins. Lightly whip cream until it just holds its shape, then whisk egg whites until they form soft peaks. Gradually beat the egg yolks into the melted chocolate until smooth. Stir half the cream into chocolate mixture, then fold in the rest. Stir in a quarter of the egg whites, then carefully fold in the remainder with a metal spoon. Set aside.
For the white chocolate mousse, melt 200g white chocolate in the same way as above and cool. Beat in the cream until well combined. Finish by whisking the mixture with an electric hand whisk for 10-15 secs, until it reaches the consistency of thick custard (take care not to over-whisk).
To assemble, pour a third of the white chocolate mousse into a 1½ litre glass bowl. Spoon over all of the dark chocolate mousse, then top with the remaining white chocolate mousse. Use a knife to swirl the dark and white layers together. Chill for at least 4 hrs.
Use the leftover chocolate to make shavings with a vegetable peeler and chill until needed. Remove the mousse from the fridge 15 mins before serving and scatter with the shavings. This can be made the day before and chilled until ready to serve.