Put gelatin and cold water in top of double boiler. Let gelatin soften. Put over boiling water and stir until gelatin is dissolved. Add sugar, stir until dissolved. Put this mixture in a large mixing bowl with the corn syrup and vanilla. Beat with electric mixer about 15 minutes, or until thick and marshmallow like. Combine cornstarch and sugar. Spread 1/3 of this mixture in the bottom of a 7x10x1 1/2" pan. Pour in marshmallow mixture, smooth with a spatula. Let cool at room temperature, about 1 hour. Run spatula around edges and turn out onto board spread with 1/3 of cornstarch/sugar mixture. Cut into squares, roll in remaining cornstarch/sugar mixture.