In a small bowl, combine the sugar and cornstarch. Use a fork to mix them together until they're well combined..
In a large mixing bowl, beat the egg whites for 1 minute at a medium-high speed. A stand or hand-held mixer may be used.
Add the cream of tartar and salt and beat until blended.
Begin adding the sugar-cornstarch mixture to the egg whites, one tablespoon at a time, until the egg white mixture is glossy and has formed stiff peaks. Be careful not to overbeat, but make sure all the sugar is dissolved. Beat in the vanilla.
Line a baking sheet with parchment paper and gently spread the meringue mixture into a 7-inch circle. Make a slight indentation in the center of the meringue where you will later place the filling.
Bake for 1 hour, 30 minutes or until barely pale golden and the outside has formed a crust. Turn the oven off and let the meringue stand in the oven with the light on for 12 hours.
When ready to serve, whisk together the lemon curd and sour cream. Spoon into the center of the cooled meringue. Top with berries and lemon zest.
Notes
Make sure the mixing bowl, beaters (or whisk), and spatula are spotlessly clean (even the tiniest bit of oil or grease can cause the meringue to fall or not rise).
Have the egg whites at room temperature. Cold white won’t whip up as well.
It’s possible to store leftovers in a well sealed container for a short time, but this dessert is best enjoyed the day after baking it.