Toast and Prepare the Hazelnuts: Preheat your oven to 180°C (160°C fan). Spread the hazelnuts on a small baking tray and toast for 8-10 minutes, or until they are fragrant and the skins are blistered. Let them cool slightly, then tip them into a clean tea towel and rub them vigorously to remove most of the skins. Set a few aside for garnish, and place the rest in a small food processor. Pulse until they form a coarse, sandy powder.

Melt the Chocolate: Place the chopped chocolate and butter in a heatproof bowl set over a saucepan of gently simmering water (a bain-marie). Make sure the bottom of the bowl doesn't touch the water. Stir occasionally until the chocolate is smooth and completely melted. Remove from the heat and set aside to cool for about 5 minutes.

Prepare the Egg Yolks: Separate your eggs, placing the whites in a large, clean bowl and the yolks in a separate medium bowl. Add the caster sugar and the pinch of salt to the egg yolks and whisk with an electric mixer (or by hand) for 2-3 minutes until pale, thick, and creamy.

Combine Chocolate and Yolks: Pour the slightly cooled melted chocolate into the egg yolk mixture and stir until fully combined. It will become quite thick at this stage. Stir in the ground hazelnuts.

Whip the Cream: In a separate bowl, whip the cold double cream until it holds soft peaks. Be careful not to over-whip it, or it will become grainy.

Whip the Egg Whites: Using clean, dry beaters, whisk the egg whites until they form stiff, glossy peaks. You should be able to turn the bowl upside down without the whites sliding out. Understanding the stages of whipping egg whites is key, and Serious Eats has a great visual guide.

Fold it All Together: This is the crucial step! What works best for me is using a large metal spoon or spatula. Add about a third of the whipped cream to the chocolate mixture and stir it in to lighten the base. Then, gently fold in the remaining cream. Next, take about a third of the egg whites and fold them in, followed by the rest of the whites in two more additions. Use a gentle cutting and lifting motion to combine everything without knocking out the air.

Chill to Perfection: Carefully spoon the mousse into 6 small glasses, ramekins, or bowls. Cover with cling film and place in the fridge to chill for at least 4 hours, or ideally overnight, until firm.

Serve: When ready to serve, garnish with the reserved toasted hazelnuts, some chocolate shavings, or a dollop of whipped cream.