Place the egg white in the bowl of an electric mixer and whisk until stiff peaks form. Gradually add the sugar, whisking well, until the mixture is stiff and glossy. Add the cornstarch and vinegar and whisk until just combined. Shape the mixture into an 18cm round on a baking tray lined with parchment paper.
Reduce oven to 250°F and bake for 1 hour 20 minutes. Turn the oven off and allow the pavlova to cool completely in the oven.