Line an 8-inch (20 cm) square baking pan with plastic wrap and oil lightly.
In a small saucepan, combine the water and vanilla. Sprinkle the gelatin over the water and let bloom for 5 minutes. Add the sugar and heat gently, stirring until the sugar and gelatin have dissolved. Let cool for 5 minutes.
Pour into a bowl and add the corn syrup. Beat with an electric mixer until soft peaks form, about 10 to 12 minutes. Spread into the prepared pan and refrigerate for 2 hours or until the marshmallow has set.
In a bowl, whisk together the cornstarch and icing sugar.
Sprinkle a work surface and a knife with some of the cornstarch mixture. Unmould the marshmallow. Cut into 25 large cubes and coat with the starch mixture. Shake to remove any excess.
If desired, caramelize the marshmallows using a small culinary torch or around a campfire. Serve immediately.
Replace the water with a fruit purée of your choice for marshmallows with different colours and flavours.
Starch and sugar-coated marshmallows (not caramelized) will keep for 1 to 2 weeks at room temperature in an airtight container.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.