Melt the Chocolate: Place the finely chopped dark chocolate 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 completely melted and smooth. Remove from the heat and set aside to cool for about 10 minutes, until it's lukewarm.

Prepare the Eggs: Carefully separate the eggs, placing the yolks in one large mixing bowl and the whites in another spotlessly clean, grease-free mixing bowl. This is a crucial step; any trace of yolk in the whites will prevent them from whipping properly.

Make the Chocolate Base: Add 50g of the caster sugar and the vanilla extract to the egg yolks. Using an electric hand whisk, beat for 2-3 minutes until the mixture is pale, thick, and has doubled in volume. This process of whipping yolks with sugar is what gives the mousse its rich, custardy foundation.

Combine Chocolate and Yolks: Gently pour the slightly cooled melted chocolate into the egg yolk mixture. Use a spatula to fold it in until just combined. The mixture will be thick and glossy. Don't overmix.

Whip the Egg Whites: Add the pinch of salt to the egg whites. Using clean beaters, whisk the egg whites on a medium-high speed until they form soft peaks. Gradually add the remaining 50g of caster sugar, a tablespoon at a time, continuing to whisk until the whites are stiff and glossy. You should be able to turn the bowl upside down without them sliding out. You can learn more about the science of egg whites on food history sites like Wikipedia.

Whip the Cream: In a separate bowl, whip the cold double cream until it holds soft peaks. Be careful not to overwhip, or it will become grainy. You want it to be thick but still have a soft, billowy texture.

Fold it All Together (The Key Step): Take about a third of the whipped egg whites and fold them into the chocolate mixture to lighten it up. Then, gently fold in the remaining egg whites in two more additions. I find that using a large metal spoon and a figure-of-eight motion works best to keep the air in.

Finish with Cream: Now, gently fold the softly whipped cream into the chocolate mixture in two stages. Continue with the same gentle folding motion until there are no more white streaks. The final mousse should be airy, even in colour, and uniform.

Chill to Perfection: Carefully spoon or pipe the mousse into six small glasses, ramekins, or dessert cups. Cover each one with cling film and place them in the refrigerator to chill and set for at least 4 hours, or preferably overnight.

Garnish and Serve: Just before serving, remove the Chocolate Mousse Cups from the fridge. Garnish with fresh raspberries, a few delicate chocolate shavings, or a dusting of cocoa powder.