Preheat the oven to 110C/100C Fan/Gas 2. Line a large baking tray with foil/parchment paper.
Melt the dark chocolate in a small bowl in the microwave - do this in 20 second intervals to prevent the chocolate from burning. Once melted, set aside to cool.
In a large bowl or the bowl of a stand mixer, beat the egg whites on high speed until soft peaks form.
Gradually add in the sugar - a tablespoon at a time, until all combined. Then add the white wine vinegar and keep mixing on high speed until stiff peaks form. If you are using a stand mixer, be sure to stop the mixer and fold the mixture with a metal spoon to ensure no liquid is falling to the bottom.
Once stiff peaks form (should take a few minutes), stop the mixer. Use a tablespoon to dollop about 6 large mounds of meringue mixture onto the baking tray.
Use two chopsticks (or similar): dip into the chocolate, then swirl into the meringue - you don't want too much but just enough for a little chocolate swirl.
Once you've swirled all the meringues, bake for 1 hour. Once baked, turn off the oven, open the door a little and leave for about 30 mins- 1 hour before serving!