Pavlova with Lemon Curd and Berries

Preheat oven to 300 °F with rack in middle. Trace an approximately 7-inch circle on a sheet of parchment paper. Turn parchment over and put on a baking sheet. Whisk together superfine sugar and cornstarch in a small bowl. Beat whites with a pinch of salt using an electric mixer at medium speed until they hold soft peaks. Add water (whites will loosen) and beat until whites again hold soft peaks. Increase speed to medium-high and beat in sugar mixture 1 tablespoon at a time. After all sugar has been added, beat 1 minute more. Add vinegar and beat at high speed until meringue is glossy and holds stiff peaks, about 5 minutes (longer if using hand-held mixer). Gently spread meringue inside circle on parchment, making edge of meringue slightly higher than center (the "crater" is for curd and fruit). Bake until meringue is pale golden and has a crust, about 45 minutes (inside will still be marshmallow-like). Turn oven off and prop door open slightly with a wooden spoon. Cool meringue in oven 1 hour.