Preheat oven to 110°C. Line a large baking sheet with parchment paper. Using a pencil, draw a 25cm circle on parchment; turn parchment over.
In the bowl of a stand mixer fitted with the whisk attachment, beat egg whites, vanilla extract, cream of tartar, and salt at medium-high speed until foamy, about 1 minute. Increase mixer speed to high. Add 1½ cups (300 grams) sugar in a slow, steady stream, beating until glossy, stiff peaks form, 2 to 4 minutes, depending on humidity. Add cornstarch, and beat until well combined, about 10 seconds. Spoon meringue onto circle on parchment. Using the back of a large spoon, spread meringue into a 10-inch disk, pressing in center to form a well with a raised 2-inch border. (Meringue will expand during baking.)
Bake until set and dry, 1½ to 2 hours. (Meringue will be very lightly colored and should feel dry to the touch, not tacky or sticky.) Turn oven off, and let meringue stand in oven with door closed for 1½ hours. Remove from oven, and let cool completely on pan.
In a large bowl, toss together strawberries, blackberries, blueberries, and ¼ cup (50 grams) sugar. Let stand for 15 minutes.
In the bowl of a stand mixer fitted with the whisk attachment, beat mascarpone, cream, vanilla bean paste, and remaining ¼ cup (50 grams) sugar at medium-high speed until soft peaks form. (Do not overbeat.)
To serve, place meringue on a serving platter. Fill center with mascarpone mixture, and top with berries. Serve immediately.