Prepare a 9 x 13 pan by lining it with plastic wrap and spraying the bottom and sides with cooking spray.
Pour ½ cup cold water in the bowl of a standing mixer. Add 2 teaspoons vanilla. Sprinkle three packets of unflavored gelatin over the water and vanilla and let sit for 10-15 minutes.
In a medium saucepan, combine two cups sugar, 1/4 teaspoons salt, ¼ cup water, and ¾ cup light corn syrup. Cook over medium heat, stirring occasionally, until it comes to a full boil. Allow it to boil one minute, then remove it from the heat and pour over the gelatin mixture.
Beat on high speed for 10 minutes (it will become very white and glossy). Pour into prepared 9 x 13 pan. Smooth top with a spatula coated with cooking spray. Cover with plastic wrap (don’t let plastic wrap touch the surface of the marshmallows, or it will stick) and let it sit for 4-6 hours at room temperature.
Sprinkle a generous layer of powdered sugar onto a baking sheet, then invert marshmallow slab onto the sugar and coat both sides well. Cut into squares using scissors or cut into shapes using cookie cutters, coating the cut sides of marshmallows with powdered sugar as well.
Store marshmallows in an airtight container.
Variation: Add peppermint extract instead of vanilla, and dredge slab in a mixture of powdered sugar and crushed candy canes.