Preheat the oven to 180°C and line a tray with parchment paper marked with a 23cm circle.
Mix the sugar with the instant espresso powder in a small bowl.
Whisk egg whites with a pinch of salt to soft peaks, then gradually whisk in the sugar mixture.
Fold the cornflour and vinegar into the firm meringue using a metal spoon.
Shape the meringue into a flat-topped circle on the prepared baking tray.
Bake at 150°C for one hour, then leave in the switched-off oven to cool completely.
Invert the cooled meringue onto a plate and carefully peel off the parchment paper.
Spread whipped double cream over the top and dust with sieved cocoa powder.