1. Preheat oven to 250ºF. Position rack in center of oven. Draw a 7-inch circle on parchment paper. Place paper, circle side down, on a baking sheet.
2. For meringue, in a large mixing bowl, beat egg whites with an electric mixer on medium until soft peaks form (tips curl). Add superfine sugar, 1 tablespoon at a time, beating on high until stiff peaks form (tips stand straight) and sugar is almost dissolved. Sprinkle cornstarch, vinegar and vanilla over mixture; fold in gently. Spoon meringue over circle on paper; build up sides to form a shell. Bake for 60 to 70 minutes or until the outside is dry.
3. Turn oven off, leaving meringue in oven with door slightly open until cooled completely (1½ to 2 hours). As it cools, the outside may crack slightly.*
4. For berry filling, about 30 minutes before serving, in a medium bowl, stir together berries and 3 tablespoons sugar. Let stand for 15 minutes.
5. For mascarpone cream, in a medium bowl combine mascarpone cheese, whipping cream, ¼ cup sugar and lemon zest. Beat with an electric mixer on medium-high until soft peaks form.
6. Spoon mixture into meringue shell. Arrange berry mixture on top. If desired, garnish with mint leaves. Serve immediately.

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*Note: You can store the meringue shell in an airtight container at room temperature for up to 2 days or in the freezer for up to 6 months.

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