1. In small bowl, with mixer at high speed, beat egg whites and cream of tartar until soft peaks form; gradually sprinkle in sugar, 2 tablespoons at a time, beating well after each addition until sugar dissolves and whites stand in stiff, glossy peaks. Beat in vanilla extract.
2. Preheat oven to 200°F. Line large cookie sheet with foil. Onto cookie sheet, spoon meringue to make 6 mounds. Spread each mound into a 4-inch round. With tablespoon, shape each meringue round to form a nest.
3. Bake 3 1/2 hours or until meringues are crisp but not brown. Cool completely on cookie sheet on wire rack. If not using right away, store shells in tightly covered container.