Toast and Prepare the Hazelnuts: Preheat your oven to 180°C (160°C fan). Spread the blanched hazelnuts on a baking tray and toast for 8-10 minutes, or until they are fragrant and lightly golden. Keep a close eye on them as they can burn quickly. Let them cool completely. Set aside about 12-15 of the best-looking nuts for garnish, and place the rest into a food processor. Blitz until you have a fine, sandy powder.

Melt the Chocolate Base: Place the chopped dark chocolate and cubed butter in a heatproof bowl set over a saucepan of simmering water (a bain-marie). Make sure the bottom of the bowl doesn't touch the water. Stir occasionally until smooth and fully melted. Remove from the heat and set aside to cool for about 10 minutes. It should be tepid, not hot.

Prepare the Egg Yolks: In a separate large bowl, use an electric mixer to whisk the egg yolks with 50g of the caster sugar until the mixture is pale, thick, and has doubled in volume. This usually takes about 3-4 minutes. Whisk in the vanilla extract.

Combine Chocolate and Yolks: Gently pour the slightly cooled chocolate mixture into the egg yolk mixture. Add the ground hazelnuts and fold everything together with a spatula until just combined. The mixture will be quite thick at this point.

Whisk the Egg Whites: In a clean, dry bowl, whisk the egg whites with a pinch of salt until they form soft peaks. Gradually add the remaining 25g of caster sugar and continue to whisk until the whites are glossy and form stiff peaks.

Whip the Cream: In another bowl, whip the double cream until it holds soft, pillowy peaks. Be careful not to over-whip, as this can make the final mousse grainy.

Fold it All Together: Now for the most important part. Take about a third of the whisked egg whites and fold them into the chocolate mixture to lighten it up. Then, gently fold in the remaining egg whites in two batches. What works best for me is a large metal spoon, using a gentle cutting and lifting motion to preserve the air. For a detailed guide on this crucial step, BBC Good Food has an excellent video tutorial on folding technique.

Finish with Cream: Finally, gently fold the softly whipped cream into the mixture until there are no more white streaks. Be careful not to overmix and knock out the air.

Chill the Mousse: Carefully spoon the mousse into 6 small glasses, pots, or ramekins. Cover with cling film and place in the refrigerator to chill and set for at least 4 hours, or preferably overnight.

Garnish and Serve: Just before serving, roughly chop the reserved toasted hazelnuts and sprinkle them over the top of each mousse.