Preheat the oven to 275°F Lightly grease a 9-inch springform pan and line both the bottom and sides with parchment paper.
Stir the ground hazelnuts with 1/3 cup of the sugar and the cornstarch and set aside.
Whip the egg whites and cream of tartar until foamy, then slowly pour in the remaining 2/3 cup of the sugar while beating and continue to whip the whites until they hold a stiff peak when the beaters are lifted (the whites will be thick and glossy).
Fold the hazelnut mixture into the whipped whites, then fold in the vinegar and vanilla. Spoon the meringue into the prepared pan and use your spatula to spread it into a tart shape, spreading the meringue fully up the sides of the pan and an inch in thickness (it will shift a little when it bakes, but ultimately holds its shape).
Bake for 90 to 120 minutes, until the meringue just starts to turn light brown and fills firm to the touch.
Cool the meringue for 15 minutes, then gently pop the latch and carefully remove the springform ring. Cool completely and then peel away the parchment paper. If the paper does not peel away easily, return the meringue to the 275F oven (without the springform ring) for 10 ti 15 minutes just to dry a little more.
Prepare the rhubarb by stirring the fruit with the sugar and the scraped seeds from the vanilla bean in the saucepot, and simmer on low heat, stirring occasionally until the rhubarb is tender, about 1 minutes. Stir the cornstarch with 2 tbsp of cold water and stir this into the rhubarb. Return the mixture to a simmer, stirring gently until thickened. Then remove from the heat and chill until ready to assemble.
Whip the cream until it holds as soft peak, then stir in the sugar and vanilla and chill until ready to assemble.
The pavlova should be assembled immediately before serving. Carefully place the meringue onto a platter, then dollop the Chantilly cream in the centre and spread gently. Spoon the rhubarb over the cream and serve. Slice and serve as you would a cake or tart.