• Preheat the oven to 135°C. Line a baking tray with baking paper and set aside.
• Place the sugar into a frying pan, shaking it out into an even layer. Place over low heat and leave to warm while you prepare the other ingredients. You don't want the sugar to melt, just to warm up slightly so that it dissolves into the egg whites more easily.
• Finely chop the dark chocolate and lightly crush the cornflakes.
• Wipe down the metal bowl and whisk attachment of a stand mixer with white wine vinegar to remove any traces of fat. Place the egg whites in the bowl, along the fine sea salt.
• Whisk the egg whites on high speed, until they reach soft peaks.
• Once all of the sugar has been added, turn off the mixer and use a spatula to scrape down the sides of the bowl. Then turn the mixer back on to high speed and whisk the meringue until stiff peaks form. When it's ready, you should be able to rub a small amount of the meringue between your fingers without feeling any sugar crystals.
• Fold the vanilla extract into the meringue, then add the chopped dark chocolate and crushed cornflakes and fold again, until evenly distributed.
• Place tablespoon-sized dollops of the meringue mixture onto the prepared baking tray. Once you have used up all of the mixture, place the tray into the oven and bake for 25-30 minutes, until the surface of the meringues feels crisp and the cookies hold their shape.
• Allow the meringues to cool completely on the tray before serving/decorating. The cookies can be stored in an airtight container at room temperature for up to 2 weeks.

To decorate (optional):

• Drizzle the cooled meringue cookies with the melted dark chocolate, then sprinkle with coarse sea salt (see Note 1). Allow to sit at room temperature until the chocolate has set, then serve.