How to Make It
Preheat the oven to 325°. Line a large baking sheet with parchment paper or foil. In a large bowl, beat the egg whites at medium speed until soft peaks form. Beat in the superfine sugar, I tablespoon at a time, and continue beating until the meringue is thick, glossy and stiff. Quickly beat in 2 teaspoons of the lemon juice, the cornstarch and the vanilla. Spoon the meringue into 6 evenly spaced mounds on the parchment-lined sheet. Using the tip of the spoon, make a well in each meringue.
Bake the meringues for 5 minutes, or until slightly dry and set. Reduce the temperature to 225°. Bake for 1 hour. Turn off the oven and leave the meringues in the oven for I hour with the oven door closed. The meringues should be perfectly white and crisp on the outside but chewy inside.
Toss the mango slices and blueberries with the remaining 1 teaspoon of lemon juice and sprinkle with superfine sugar if desired. Set the meringues on dessert plates. Whip the heavy cream and spoon onto the meringues. Top with the mango slices and blueberries, sift confectioners' sugar over the tops and serve at once.