Line baking sheets with cooking parchment or buttered and floured foil. Draw six 3-inch-diameter circles on the parchment at least 3 inches apart.
Mix 1/3 cup sugar with the cornstarch; set aside.
With an electric mixer, beat egg whites and cream of tartar on high speed until soft peaks form. Gradually add remaining sugar, 1 tablespoon at a time, beating well after each addition. Then gradually beat in cornstarch-sugar mixture until whites are stiff and glossy. Beat in vinegar and vanilla until blended. Mound mixture evenly on circles. With the back of a large spoon, make a bowl-like indentation in the center of each mound.
Bake in a 300° oven until golden and dry to touch, 50 to 60 minutes. Cool completely on pan. If making ahead, store airtight up to 1 day. With a wide spatula, carefully transfer each meringue to a plate.
Mix fruit with liqueur; spoon mixture into the cavity of each meringue. Add whipped cream to taste. Garnish with mint.