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1
The night before serving, adjust oven rack to lower-middle position and heat oven to 500 degrees.
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2
Line a lipped cookie sheet (approximately 12 by 18 inches) with parchment paper. Trace two side-by-side rectangles - about 5 inches by 12 or 13 inches - on the paper. Crumple the paper, then smooth it out again and line the bottom of the pan.
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3
Whisk together confectioner's sugar, 1/2 cup granulated sugar and cornstarch. Set aside.
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4
In the bowl of a mixer, beat egg whites and cream of tartar until they turn from foamy to white and beaters just start to leave a trail. On low speed, add sugar mixture, a heaping tablespoon at a time. Increase speed to medium-high and beat to the consistency of marshmallow cream, 2 to 3 minutes longer.
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5
Just before forming meringues, flick water onto parchment with fingertips. Divide meringue in half, spooning two or three mounds down each traced rectangle. Use a rubber spatula, or a cake decorator's metal offset spatula, to fill in and even out rectangles.
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6
Set meringues in oven, close door and turn oven off (but leave oven light on). Let meringues dry overnight. Remove, then carefully set a wire rack or cookie sheet over meringues. Invert; peel off parchment and turn right side up. (It's OK if edges chip.) Wrap in foil and set aside until ready to proceed.
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7
No more than two hours before serving, whip cream to soft peaks, beating in remaining sugar and the vanilla. Set one meringue on a platter. Spread with half the cream and sprinkle on half the berries. Repeat with remaining meringue, cream and berries. Refrigerate, uncovered, until ready to serve.
