Separate the egg yolks and egg whites into two separate bowls. Make sure there are no traces of egg yolks in the egg whites.
Melt the chocolate in a heatproof bowl over simmering water. Once melted remove from the heat.
Whisk the egg yolks with 2 tablespoons of water over simmering water, until smooth and fluffy, and then add the melted chocolate into the egg mix.
Whisk the egg whites until you have firm peaks. Add the sugar and whisk again until you have stiff peaks.
Mix 1/4 of the egg whites into the chocolate mixture. Gently fold in the remaining egg whites making sure not to knock the air out of the mixture.
Pour the mousse into glasses and refrigerate for at least 1 hour.
Toast the almonds in a dry pan until golden brown. Scatter the almonds and the ground pistachio in a thin layer onto a silicone baking sheet.
Melt the caster sugar in a heavy bottomed pan without stirring until a light caramel is formed. Pour the caramel over the nuts and allow to cool.
Once the caramel has set, break the caramel into shards and serve on top of the chocolate mousse.
«Pasteurised egg whites work best when making mousse - you should be able to find them in most large supermarkets. If you can't get hold of them, use eggs that are a few days old (but still within their use by date!).»