Step 1
The night before serving adjust oven rack to lower-middle position and heat oven to 500 degrees.
Step 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.
Step 3
Whisk together confectioner's sugar 1/2 cup granulated sugar and cornstarch. Set aside.
Step 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.
Step 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.
Step 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.
Step 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.