Dark Chocolate Mousse

Melt the chocolate and butter together in a heatproof bowl set over a pan of barely simmering water, stirring until smooth and glossy.

Keep the base of the bowl clear of the water and the heat low. Chocolate that overheats, or that catches a few drops of water or steam, seizes into a stiff dull paste and does not come back.

Lift the bowl off the pan and leave it for 10 minutes, until the mixture is warm to the touch rather than hot.

Beat the egg yolks into the warm chocolate one at a time, letting each disappear before the next goes in. Chocolate still hot from the pan cooks the yolks into grainy threads on contact, as the dark chocolate brownies in this collection set out.

Whisk the egg whites with the fine salt in a spotlessly clean, dry bowl until they hold soft peaks that flop at the tip. A trace of yolk or grease and they will not take air at all.

Whisk in the caster sugar a spoonful at a time, until the meringue is glossy and holds a peak that stands with a slight curl at the top.

Stop there. Whites whipped past that point break into a dry curdled foam that will not fold into anything and cannot be rescued.

Stir a third of the whites into the chocolate briskly to slacken it, without worrying about the air you knock out.

Fold the rest in in two additions with a large metal spoon, cutting down through the middle and sweeping around the bowl.

Stop while a few pale streaks are still visible, since they disappear as you divide it up.

Divide the mousse between six glasses or small bowls.

Chill for at least 4 hours, until set firm enough to hold the mark of a spoon.

Take them out of the fridge 15 minutes before eating. Fridge-cold cocoa butter tastes flat and waxy, and the aroma only comes back as it warms.