Preheat the oven to 180°C/gas mark 4/350°F. Line a flat oven sheet with baking parchment and, using a 23cm/9inch cake tin, draw a circle on it.

Mix the sugar with the instant espresso powder in a small bowl and set aside.

In a clean, grease-free metal bowl whisk the egg whites and salt until they are holding soft peaks. Keep whisking while you add the sugar-coffee mixture a tablespoon at a time.

When you have a firm, ecru-coloured meringue, fold in the cornflour and vinegar using a grease-free metal spoon.

Dollop the meringue mixture inside the drawn circle. Smooth and shape it with a spatula so that it looks rather like the crown of a boater: it must be flat on top.
Put it in the oven and immediately turn down to 150°C/gas mark 3/300°F and cook for an hour. Switch off the oven and leave inside until cool.

Once the base is cool, lift it carefully in its paper and place it top-side down on a flat plate and peel off the paper.

Whip the double cream until thick but soft and spread delicately over the top of the meringue. With a teaspoon, push the cocoa through a fine sieve or tea strainer to decorate the top, cappuccino-style.