Put the broken chocolate into a bowl set over a saucepan of gently simmering water – don’t let the bottom of the bowl touch the water. Leave the chocolate to melt slowly, stirring occasionally.
Meanwhile, beat the egg yolks lightly with a fork.
When the chocolate has melted, remove from the heat and allow it to cool for a couple of minutes before stirring in the egg yolks.
Whip the egg whites until they form soft peaks and then beat in the teaspoon of caster sugar.
Now, being very patient and using a metal spoon, very slowly fold the eggs whites into the chocolate mixture. It takes a while to incorporate them all without beating the air out of the mixture.