Preheat the oven to 325°F / 170°C. Line a baking sheet with parchment paper.
Beat 150ml of room-temperature egg whites in a clean, grease-free bowl until soft peaks form. Gradually beat in 1 cup of superfine sugar, a spoonful at a time.
Beat for another 3 minutes, until thick and glossy. Rub a little between your fingers; keep beating if it feels gritty. Briefly mix in 1 tablespoon of sifted cornstarch and 1 teaspoon of white vinegar.
Shape into a dome about 8in / 20cm wide and 1½-2in / 4-5cm high, with gently sloping sides and a level top.
Put in the oven and immediately reduce the heat to 240°F / 115°C. Bake for 90 minutes without opening the door. Turn off the oven and leave closed overnight to cool.
Just before serving, whip 1½ cups of heavy cream, ¼ cup of superfine sugar, and 1 teaspoon of vanilla extract to soft peaks. Spoon onto the meringue and top with 250g of halved strawberries and 125g of blueberries.
