1. Preheat oven to 250°. Using an electric mixer, beat egg whites with cornstarch on medium speed until very foamy. Sprinkle in 3/4 cup sugar, about 1 tbsp. at a time, while beating; beat until meringue holds stiff, shiny peaks, about 15 minutes. Add vanilla and vinegar and beat 30 seconds more.
2. Line 2 baking sheets with waxed paper. Using a large spoon, drop meringue onto paper in 8 equal dollops 2 in. apart. Press back of spoon into dollops to make shallow bowls about 3 in. wide.
3. Bake meringues until a pale golden crust has formed (surface shouldn't be sticky), 40 to 50 minutes. Let cool 5 minutes, then transfer from waxed paper to a rack; cool 5 to 10 minutes more.
4. Put berries in a small saucepan with remaining 1 tbsp. sugar and bring to a simmer over medium heat, uncovered. Simmer, stirring gently every now and then, until berries begin to break down, 5 to 10 minutes. Let cool.
5. Whip cream. Top each cooled meringue with cream and berries, dividing evenly.
Make ahead: Pavlovas, up to 1 week airtight at room temperature; sauce, up to 1 day, chilled.