In a clean bowl, whisk egg whites to soft peaks, slowly drizzle in sugar, and whisk again until white and glossy.
Break up chocolate into a heatproof bowl and place over a pot filled with 2 cm of water. Let the water simmer and the chocolate melt for 5 minutes. Remove bowl from heat, and stir the chocolate with a metal spoon. Let it cool for 5 minutes, then add in egg yolk and stir.
Using a metal spoon, fold a spoonful of egg white into the chocolate to loosen up the mix. Then slowly fold in the remaining egg whites. Be sure to take your time at this stage, as you don’t want to lose the air which keeps the mousse light.
Divide into 4 serves, cover with plastic wrap, and refrigerate for at least 1 hour.
For the salted caramel, start by lining a baking sheet with foil. Warm the pistachios in an oven or microwave for about 1 minute.
Add the sugar, water and 1 tsp of salt to a heavy-based saucepan and put over medium heat, do not stir, let the sugar dissolve for about 3 minutes. Once dissolved, increase the heat and cook without stirring, swirling the saucepan gently by hand. Be patient and don’t walk away from your pan; when the syrup has turned yellow add your warmed pistachios.
Use a heatproof spatula and stir to coat the pistachios. Cook until your caramel is the colour of amber and has the aroma of toffee, then pour immediately onto a lined tray. Tap tray on bench, allowing the caramel to spread out to a thin layer before it hardens, then sprinkle with remaining teaspoon of salt flakes.
Let the salted caramel cool, break into shards and serve on top of mousse.