For the filling
Preheat the oven to 150°C, gas mark 2.
Trace a 15cm circle on a piece of baking paper and set it in a baking sheet.
Whip the egg whites and salt together until medium-stiff peaks form.
Add the water slowly while whipping the whites on a low speed.
Drizzle in the sugar, then turn up the speed and whip until stiff peaks stage.
Fold in the vinegar, vanilla and cornstarch.
Mound the meringue into the circle on the parchment.
Use a spatula to smooth the sides of the Pavlova upwards and create a crown.
Bake for 30 minutes or until the meringue starts to turn a very pale tan colour, then reduce the heat to 135°C, gas mark ½, and continue to bake for 45 minutes.
Turn off the oven (don’t open the door), turn on the light in the oven (don’t stress if the light doesn’t work) and let the meringue sit in the cooling oven for at least an hour, but it can be stored like this overnight. (The centre of the Pavlova will collapse, but that’s where you will put your filling).
To make the filling, place the vanilla, cream, mascarpone and sugar in the bowl of an electric mixer and whisk until stiff peaks form.
Pile the filling into the centre of the Pavlova and decorate with the raspberries and pomegranate rubies.
Dust with the icing sugar before serving.