Iceberg Pavlova

Procedure

1. Prep the shelf. Heat the oven to 250°F (120°C) with a rack in the middle. Trace an 8-inch (20 cm) circle on a sheet of parchment and flip it ink side down onto a baking sheet.
2. Whip the whites. In a spotless bowl, whip the egg whites and salt on medium speed until soft peaks form, 2 to 3 minutes. Keep the bowl free of fat; even a speck stops the whites from rising.⏱ 3:00
3. Dissolve the sugar. Raise the speed to medium-high and add the superfine sugar 1 tablespoon at a time, about 15 seconds apart. Keep whipping until the meringue is stiff and glossy and feels smooth, not gritty, between your fingertips, 5 to 8 minutes more.⏱ 0:15⏱ 8:00
4. Lock it in. Sprinkle the cornstarch, white vinegar and 1 teaspoon vanilla extract over the meringue and whip 10 seconds more, just to combine.⏱ 0:10
5. Shape the iceberg. Pile the meringue inside the circle and shape it with a spoon into a tall disk with a shallow crater in the center, sweeping the sides up into jagged ridges.
6. Low and slow. Bake until the outside is dry and crisp but still pale, about 75 minutes. Turn off the oven, prop the door open with a wooden spoon and let the pavlova cool inside for at least 1 hour. Slow cooling prevents cracks. Respect the thermal shock.⏱ 1:15:00⏱ 1:00:00
7. Jewel the berries. While it cools, toss the blueberries, raspberries and blackberries with the granulated sugar and lemon juice and let stand until glossy and juicy, about 15 minutes.⏱ 15:00
8. Whip the snow cream. Whip the heavy cream, mascarpone, 2 tablespoons powdered sugar and ½ teaspoon vanilla extract to soft, billowy peaks. Stop early; it keeps thickening as you spread it.
9. Assemble at the last minute. Set the pavlova on a platter, pile the snow cream into the crater, spoon the berries and their juices over the top and scatter on the mint leaves. Dust with more powdered sugar and serve within the hour.