Preheat the oven to 180 C/gas mark 4 and line a baking tray with baking parchment.
Beat the egg whites until satiny peaks form, and then beat in the sugar a spoonful at a time until the meringue is stiff and shiny. Sprinkle over the cocoa and vinegar, and the chopped chocolate. Then gently fold everything until the cocoa is thoroughly mixed inches
Mound on to a baking sheet in a fat circle approximately 23cm in diameter, smoothing the sides and top. Place in the oven, then immediately turn the temperature down to 150 C/gas mark 2 and cook for about one to one and a quarter hours.
When it’s ready it should look crisp around the edges and on the sides and be dry on top, but when you prod the center you should feel the promise of squishiness beneath your fingers.
Turn off the oven and open the door slightly, and let the chocolate meringue disc cool completely. Or, leave the meringue in the oven with its door closed overnight if you’re not serving immediately.