Mousse au Chocolat

Separate the eggs, placing yolks in a large bowl and whites in a spotlessly clean medium bowl. Finely chop the chocolate and cube the butter. Measure out all remaining ingredients. Because this mousse relies on eggs that are only gently warmed rather than fully cooked, use pasteurized eggs if you can find them — and be especially mindful serving it to pregnant women, young children, the elderly, or anyone with a weakened immune system.

Melt the chocolate and butter together in a heatproof bowl set over a pan of barely simmering water, stirring gently until completely smooth. Remove from the heat, stir in the espresso and vanilla extract, then let the mixture cool until just warm to the touch, about 5 minutes.

Whisk the egg yolks with 15g of the caster sugar until pale and slightly thickened, about 2 minutes, then stir into the cooled chocolate mixture until fully combined.

Whip the cold double cream in a chilled bowl until it holds soft, floppy peaks — it should mound gently but not be stiff. Set aside.

Add the pinch of salt to the egg whites and beat on medium speed until foamy, then gradually add the remaining 15g caster sugar and continue beating until the whites hold firm, glossy peaks that stand straight when the whisk is lifted.

Fold the whipped cream into the chocolate mixture using a large rubber spatula, cutting down through the centre and sweeping up and over until just combined — a few pale streaks are fine at this stage.

Add one-third of the beaten egg whites to the chocolate mixture and stir in firmly to loosen the base. Add the remaining whites in two additions, folding each in with a light hand until no white streaks remain and the mousse is uniformly airy.

Spoon or pipe the mousse into four serving glasses or ramekins. Cover each with cling film.

Refrigerate for a minimum of 3 hours, or overnight, until fully set and the texture is silky and just firm.

Remove from the refrigerator 10 minutes before serving to take the chill off. Serve as is, or with a small spoonful of lightly whipped cream and a few shavings of dark chocolate.