Mix the sugar, corn syrup, salt, and 1/2 C. (118 ml) water in a saucepan. Stir it over low heat until the sugar is dissolved and a syrup has formed.
Cook it until firm ball stage (244 degrees Fahrenheit about 120 C. on a candy thermometer.) Remove pan from heat, and slowly and carefully pour the syrup into the gelatin/water mixture in your mixer. Beat the mixture at high speed until thick, white, and tripled in size, approximately 15 minutes. (If you stop before this time, you will have marshmallow creme which you can store in a jar and use like the commercial stuff.) Add the vanilla and beat just long enough to incorporate it.
Dust an 8" x 12" (20 cm x 30 cm) glass baking pan with cornstarch. Pour mixture into pan, and dust the top with more cornstarch. Wet your hands and pat the mixture to smooth out the top. Dust again.
Let stand overnight to dry out, uncovered. Next morning turn
the "marshmallow cake" out onto a board, and cut in into small pieces with a
dry, HOT knife. Dust again.