To make pavlova, preheat oven to 150°C fan-forced. Line a baking tray with baking paper. Draw a 22cm circle onto paper. Using an electric mixer, beat eggwhites in a clean dry bowl until soft peaks form. Add sugar, 1 tbs at a time, beating constantly and occasionally scraping down sides of the bowl. Whisk for about 8-10 minutes until sugar dissolves and meringue is thick and glossy. Sift over cocoa and cornflour, drizzle with vinegar and gently fold until combined. Spoon mixture onto the circle on the baking paper, shaping sides to form a mound. Bake for 5 minutes. Reduce heat to 120°C fan-forced and cook for 1 hour and 15 minutes to 1 hour and 30 minutes until top is firm. Turn off the heat and leave the pavlova in the oven with the door slightly ajar until it cools (for at least 4 hours or overnight).
Meanwhile, to make cherry syrup, halve and pit 300g cherries. Refrigerate remaining cherries until ready to serve. Combine sugar and water in a small saucepan, stir over medium-high heat until sugar dissolves. Bring to the boil. Add halved cherries and return to the boil, then reduce heat and simmer for 5 minutes until tender. Place into an airtight container and chill until ready to serve.
To serve, whisk cream in a bowl until firm peaks form. Gently fold through yoghurt. Spoon mixture over pavlova. Top with reserved cherries. Drizzle with the cooled cherry syrup, sprinkle with chocolate and serve immediately.