For a dessert that is ethereally light, a meringue-based roulade ("roll") is ideal. Modeled on the concept of a Swiss roll, for this recipe we've substituted a baked meringue for the traditional sponge cake.
A simple mixture of passion fruit custard and fresh berries makes a sweet-tangy filling that balances the sweetness of the meringue nicely. The custard is made with thick Greek yogurt, and if passion fruit isn't your thing, substitute 1/3 to 1/2 cup lemon curd instead. This dessert is best served the day it is assembled.
Position a rack in the middle of the oven and preheat to 325 degrees F. Line a baking sheet with parchment, wax paper, or a silicone baking mat.
Place the egg whites in the bowl of a mixer and add a pinch of cream of tartar. Beat until the mixture holds stiff peaks. Add the sugar, one tablespoon at a time, beating on high speed until the meringue is thick and glossy. Then on low speed, mix in the cornstarch, vinegar, and vanilla.
Spread the meringue on the prepared pan, forming it into a 12 x 9-inch rectangle. Bake until the top is lightly golden, crisp, and firm to the touch, about 20 minutes. Let the meringue cool for 5 minutes.
Place another piece of parchment or wax paper on a clean work surface and dust liberally with confectioner’s sugar. Pick up the meringue (and its paper) from the baking sheet and gently place it upside down on the sugared work surface. Carefully peel off the paper lining and discard. Let the meringue cool completely at room temperature.
To make the filling, combine the cornstarch with 1-1/2 tablespoons of cold water in a small bowl and mix to a smooth paste. Meanwhile, place the passion fruit purée, 4 tablespoons of sugar, and 2 tablespoons of water in a small saucepan and bring the mixture to the start of a simmer over low heat. Stir until the sugar dissolves. Add the cornstarch mixture and cook until the sauce is thick and glossy. Remove the pan from the heat and cool for 5 minutes, then whisk in the egg yolk. Let the sauce cool completely, then stir in the yogurt. Whip the heavy cream until it is very stiff. Fold the cream into the yogurt mixture; if the custard isn’t sweet enough for your taste, add superfine sugar to adjust the sweetness. Refrigerate the custard until it’s very cold.
To assemble, spread the yogurt custard over the meringue, keeping a 1-inch border free of filling on three sides. On the long edge away from you, leave a 2-inch border uncovered. Sprinkle the strawberries and raspberries evenly over the custard, followed by the pistachio nuts, if using. Starting with the long side nearest you, carefully roll up the meringue, using the paper to help roll it.
Place about 2 tablespoons of confectioners’ sugar in a fine-mesh sieve, and sieve the sugar over the roulade. Serve immediately, or refrigerate for up to 4 hours.