Preheat the oven to 450 degrees. Spray a 9-inch square cake pan with cooking spray. Add enough cornstarch to coat the pan, tapping out the excess.

Combine egg whites, cream of tartar, and salt in a mixer. Beat on low speed until foamy then increase the speed to high and add the sugar gradually, about 1 tablespoon at a time. Beat until stiff and glossy. Beat in the lemon juice and vanilla.

Spread the mixture into the prepared pan and place in the oven. Immediately turn off the oven and let sit in the oven for at least 6 hours or overnight.

Whip the cream and spread on top of the meringue base evenly. Cover pan and refrigerate at least 2 hours.

When ready to serve, combine the berries with the lemon zest and spread on top of the cream. Cut into squares and serve.