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.
Stir together sugar, cornstarch, and salt in a 2-quart heavy saucepan, then add lemon juice and butter. Bring to a simmer over medium-high heat, whisking, then continue to simmer, whisking constantly, 1 minute. Lightly beat yolks in a small bowl and whisk in 1/4 cup lemon mixture, then whisk into remaining lemon mixture in saucepan. Reduce heat to low and cook, whisking constantly, until curd is thickened, about 2 minutes (do not let boil).
Transfer to a bowl and stir in zest. Chill, surface covered with parchment, until cool, about 1 1/2 hours.
Beat heavy cream until it just holds stiff peaks, then fold 1/4 cup beaten cream into curd to lighten. Spoon lemon curd into meringue and mound berries on top.
Serve remaining whipped cream on the side.
•For best results, keep oven door closed as much as possible during baking.•Meringue can be made 2 days ahead and frozen, wrapped well in plastic. Thaw before serving.•Curd can be made 2 days ahead and chilled.
