Marshmallows were originally made from the root of the marshmallow plant; today, corn syrup and sugar are the main ingredients. Homemade ones can be cut into any shape you like. Combine gelatin and 1/2 cup cold water in the bowl of an electric mixer with whisk attachment. Let stand 30 minutes. Combine granulated sugar, corn syrup, salt, and 1/2 cup water in a small heavy saucepan; place over low heat, and stir until sugar has dissolved. Wash down sides of pan with a wet pastry brush to dissolve sugar crystals. Clip on a candy thermometer; raise heat to high. Cook syrup without stirring until it reaches 244B0 (firm-ball stage). Immediately remove pan from heat. With mixer on low speed, slowly and carefully pour syrup into the softened gelatin. Increase speed to high; beat until mixture is very thick and white and has almost tripled in volume, about 15 minutes. Add vanilla; beat to incorporate. Generously dust an 8-by-12-inch glass baking pan with confectionersb sugar. Pour marshmallow mixture into pan. Dust top with confectionersb sugar; wet your hands, and pat it to smooth. Dust with confectioners sugar; let stand overnight, uncovered, to dry out. Turn out onto a board; cut marshmallows with a dry hot knife into 1 1/2-inch squares, and dust with more confectioners sugar.