Frozen Meringue Cake with Seasonal Berries

Preheat oven to 200 °F. Line 2 large baking sheets with aluminum foil; butter and flour foil. In large bowl using electric mixer, beat egg whites until soft peaks form. Gradually add sugar, beating until stiff and glossy. Drop 8 (approximately 1/3-cup) mounds of meringue about 1 inch apart onto baking sheets and use spoon to make indentations in centers of mounds. Bake in upper and lower thirds of oven until crisp but still white, about 45 minutes. Turn oven off and cool meringues in oven 1 hour. Using metal spatula, transfer meringues to rack to cool completely. (Meringue shells can be made up to 5 days ahead and kept in airtight container at room temperature.)