Using a pastry brush, dust the cornstarch mixture off the marshmallow. Reserve the cornstarch mixture. Cut the marshmallow into long strips, and roll the strips into the reserved cornstarch mixture, then roll the strips into a spiral, to serve or store. Alternatively, cut the marshmallow into the shape of your choice. Dust the shapes in the cornstarch mixture to keep the pieces from sticking together, dusting off any excess. The marshmallows keep well for one two weeks in an airtight container, stored in a cool, dry place. Reserve the cornstarch mixture. Store any excess cornstarch mixture in an air-tight container for a future use.