Butter a 9x13-inch baking pan. Mix together powdered sugar and cornstarch in a small bowl. Coat buttered pan with 1/3 of the sugar mixture, shaking to cover sides as well as the bottom of the pan. In a saucepan, combine granulated sugar, gelatin and salt. Stir in the cold water and let stand for 5 minutes, until gelatin softens. Cook over low heat, stirring constantly, until sugar dissolves. Do not boil. Remove from heat and cool to room temperature. Transfer mixture to electric mixer bowl. Add vanilla, then beat at high speed until mixture is thickened and forms soft peaks (about 10 to 15 minutes). Pour mixture into prepared pan, spreading evenly. Dust top with half of remaining powdered sugar mixture. Cover loosely and let stand at room temperature for 2-8 hours. Remove from pan and cut into 1-inch squares or other shapes. (Dip knife or cutter in cold water to prevent sticking.) Roll marshmallows in remaining powdered sugar mixture. Store in an air-tight container at room temperature up to 2 months. Adapted by Karen Mintzias, from a recipe in: "The Cooks Book of Uncommon Recipes" by Barbara Hill Posted to MM-Recipes Digest V4 #8 by "Rfm" on Feb 16, 99