Put the espresso, custard, and chopped chocolate in a pan and stir together over a gentle heat. When the chocolate has melted, remove from the heat and scrape into a large mixing bowl to cool.
Once cool, use another mixing bowl and electric beaters to beat the egg whites to stiff peaks. Add a pinch of salt and the sugar, and beat again for a minute until stiff.
Stir 1/3 of the beaten egg whites into the custard mixture until no streaks remain, then gently fold in the rest of the egg whites with a large metal spoon. Try to keep in as much air as possible.
Divide between 4-6 small ramekins or pots, shaking gently to flatten the tops, then chill for at least 4 hours, but preferably overnight. Dust with cocoa when you’re ready to serve – with extra shots of hot espresso on the side.