1. For a 1.2 litre terrine, cut a strip of grease-proof paper as long as the tin and wide enough to cover the bottom and come up with both long side, with about 7.5 cm to spare. Put the paper in the tin as if to line it, then draw a line across the paper on both sides where it meets the bottom edge, and the top edge of the tin. Take the paper out of the tin and draw two lines at equal intervals between the bottom and top edge lines you have already drawn. (This will help you make each layer exactly the same depth when filling the tin.) Lightly grease the paper and use to line the tin.
2. Whip a third of the cream to just soft peak stage, then chill. Melt the milk chocolate in a heatproof bowl set over a pan of simmering water with a hand-held electric blender for about 2-3 minutes until the misture has at least doubled in volume. Once you have reached this stage, remove the bowl from heat and keep whisking, adding the melted chocolate, but also cools the mixture more quickly. Leave to cool for a few minutes, then carefully fold in the whipped cream.
3. Quickly whisk two egg whites into soft peaks, making sure the bowl and whisk are spotless to ensure the maximum volume is obtained. Using a large metal spoon, carefully fold the egg white into the chocolate mixture so you don't knock out too much air.
4. Pour mixture into the terrine, filling it to first mark (any extra can be frozen), then freeze for 20-30 minutes before starting the white chocolate mixture. Repeat process with white chocolate, then the plain; cover with plastic film and freeze completely. To guarantee a good set, make terrine at least 8-10 hours or up to 2 days before.
5. Dip the tin briefly in a large bowl of hot water. Put a chopping board or flat plate on top and invert the mousse. Gently remove the paper and open freeze for about 10 minutes to re-set. To decorate,pipe an outline of melted plain chocolate in a teardrop shape into each plate. Dust the plates lightly with icing sugar, then carefully fill the teardrop with cream. Slice the mousse, wiping the knife after each cut, and put a slice on each plate. Decorate with chocolate curls and serve at once.
