Melt the chocolate and butter. Place the chopped chocolate and cubed butter in a large heatproof bowl set over a pan of gently simmering water — ensure the bowl does not touch the water. Stir slowly with a silicone spatula until the mixture is completely smooth and glossy; you'll see it transform from a pile of rough chunks into a shimmering, dark river. When you lift the spatula, the chocolate should fall in a thick, even ribbon. Remove from the heat and let it cool for 5 minutes.

Whip the egg whites to stiff peaks. In a clean, dry bowl, whisk the egg whites with a pinch of salt using an electric mixer on medium speed. As they begin to foam, the mixture will turn from clear to opaque white, and you'll hear the sound change from a slosh to a soft hiss. Gradually add 25g of the caster sugar, a spoonful at a time, and continue whisking until the peaks stand up tall and firm when you lift the beaters — the surface should look like a glossy, snow-white cloud that doesn't collapse.

Whip the cream to soft peaks. In another bowl, pour the very cold double cream, the remaining 25g of caster sugar, and the vanilla bean paste. Whisk on medium-high speed until the cream thickens and forms soft, droopy peaks. You'll know it's ready when the whisk leaves visible trails in the cream, and if you tilt the bowl, the cream moves very slowly but doesn't slide out. It should feel pillowy and light, not stiff or buttery.

Fold the cooled chocolate into the cream. Pour the slightly warm melted chocolate mixture into the bowl of whipped cream. Using a large flexible spatula, cut down through the centre and sweep up the side, rotating the bowl as you go. Don't stir — you want to fold gently until you see no more streaks of white cream. The mixture will look like a uniform, pale brown mousse at this stage, and it should be thick but still pourable.

Fold in the egg whites. Add one-third of the whipped egg whites to the chocolate-cream mixture and fold together with the same gentle motion. This first addition will loosen the mixture, making it easier to incorporate the rest. Add the remaining egg whites in two more batches, folding just until no white streaks remain. The final mousse should be a deep, even brown, airy and billowy — it will feel almost weightless in the bowl.

Chill and set. Divide the mousse evenly among six serving glasses, ramekins, or small bowls. Gently tap each glass on the counter to release any trapped air bubbles. Cover each with cling film and refrigerate for at least 4 hours, though overnight is ideal. The mousse will set to a firm, wobble-like consistency — when you press the top gently with a finger, it should spring back slightly and leave no indentation.