Preheat the oven to 180°C/160°C Fan/gas mark 4/350ºF 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 in. Mound on to a baking sheet in a fat circle approximately 23cm / 9 inches in diameter, smoothing the sides and top. Place in the oven, then immediately turn the temperature down to 150°C/130°C Fan/gas mark 2/300ºF 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 centre you should feel the promise of squidginess beneath your fingers. Turn off the oven and open the door slightly, and let the chocolate meringue disc cool completely.When you're ready to serve, invert on to a big, flat-bottomed plate. Whisk the cream till thick but still soft and pile it on top of the meringue, then scatter over the raspberries. Coarsely grate the chocolate so that you get curls rather than rubble, as you don't want the raspberries' luscious colour and form to be obscured, and sprinkle haphazardly over the top, letting some fall, as it will, on the plate's rim.