Preheat oven to 130°C fan forced. Draw a 22cm circle on a sheet of non-stick baking paper and place, marked side down on a large greased baking tray.
Whisk the eggwhites on high speed in an electric mixer until soft peaks form. Gradually add the sugar, 1 spoon at a time, whisking until meringue is thick and glossy. After the last spoon, continue to beat on high speed a further 3 minutes. Fold in the vinegar then cornflour. Fold in 1/2 cup of the pistachios.
Drop large spoonful™s of the meringue on the tray using the circle as a guide. Sprinkle over half the pistachio. Bake for 1 hour 20 or until the top feels crisp and dry. Turn the oven off, leave the pavlova to cool completely in the oven with the door closed.
Just before serving, slide the pavlova wreath on a serving plate. Swirl the cream and hazelnut spread together. Spoon over the pavlova. Cut the cheeks from the mangoes. Using a large spoon, remove the mango fruit from the cheeks, slice. Arrange over the cream. Sprinkle with remaining pistachio and grated chocolate if desired. Serve.