Eton Mess is a lightly sweet, old-fashioned British dessert made by blending whipped cream with freshly crushed berries and crisp meringue. As a shortcut, use store-bought meringues instead of homemade.
Preheat the oven to 225°. Line 2 large baking sheets with parchment paper. In a standing mixer fitted with a whisk or using a handheld electric mixer, beat the egg whites to soft peaks. With the mixer on, gradually add the granulated sugar and beat at medium speed until stiff, glossy peaks form, about 5 minutes. Reduce the speed to low and beat in the cornstarch and vinegar. Increase the speed to high and beat for 20 seconds. Spread the meringue on the prepared baking sheets in 9-inch rounds about 1/2 inch thick. Bake for about 3 hours, until the meringue is dry throughout and crisp. Let the meringue cool completely.
In a bowl, whip the cream to soft peaks. Add 1/2 cup of the confectioners' sugar and the vanilla seeds and whip to firm peaks.
In a large bowl, coarsely mash three-quarters of the raspberries with the remaining 1/4 cup of confectioners' sugar. Leave the remaining berries whole. Fold the mashed raspberries into the whipped cream. Coarsely crumble the meringue and fold all but 1 cup into the whipped cream mixture. Spoon the Eton Mess into bowls and top with more crumbled meringue, the remaining raspberries, the pistachios and the mint leaves.
The meringue can be stored in an airtight container overnight.