Line large baking sheet with parchment paper or greased foil; using 8-inch round cake pan as guide, trace circle. If using paper, turn over. Set aside.
In stainless steel or glass bowl, beat egg whites until soft peaks form. Beat in sugar, 2 tbsp at a time, until stiff glossy peaks form. Beat in cornstarch and vinegar, then vanilla. Fold in chocolate.
Spoon egg white mixture onto circle on baking sheet; spread to fill evenly. With back of spoon, form nest shape with hollow about 5 inches in diameter and rim about 1 inch higher than hollow, spreading a little beyond drawn circle if desired.
Bake in centre of 275°F oven until crisp on the outside but still soft on the inside, about 1 hour. Turn off oven; let meringue cool in oven for 1 hour. Slide metal spatula under meringue to loosen from paper; slide onto flat serving plate. (Make-ahead: Let stand at room temperature for up to 2 hours.)
In chilled bowl, whip cream; spoon into meringue nest, leaving raised rim bare. Arrange berries on top of cream. To serve, cut with serrated knife.