Prep: Combine cornstarch and confectioners sugar in a small bowl. Spray a 13x9" dish with non-stick spray, then dust it with the cornstarch mixture. You will have some starch/sugar left over - save it to dust the top of the marshmallows. 

Step 1: In a large mixing bowl (I used my electric mixing bowl), combine gelatin and 1/2 cup cold water. Whisk to dissolve the gelatin completely, allow to sit while you prepare the rest of the recipe. 

Step 2: Combine granulated sugar, salt, corn syrup, and the remaining 1/2 cup water in a saucepan. Over medium heat, stir the ingredients to dissolve the sugar. Once dissolved, turn the heat to high and bring to a boil - using a candy thermometer, closely monitor the temperature. 

Step 3: Whisk the mixture as it boils and keep a close eye on the temperature. Do not be tempted to step away from the mixture - it can burn quickly! As soon as it reaches 240 degrees F remove it from the heat. This takes about 15 minutes. 

Step 4: Once the mixture reaches 240 degrees F, slowly pour it into the dissolved gelatin mixture. Use an electric mixer to whip the mixture for about 15 minutes - use your whisk attachment for best results. It will become light and fluffy...like marshmallows! Note: this is easiest if done using an electric mixing stand - set it to a medium speed, and let it mix! If you are using a hand mixer, your arm is probably going to get a bit tired!

Step 5: Once the mixture becomes marshmallow-like, add the vanilla (or extract of choice) and mix until combined.  

Step 6: Use a spatula to pour the marshmallow mix evenly into the prepared baking dish. Dust with remaining confectioners sugar mixture, and let the marshmallows sit overnight, uncovered. Once the mallows have dried out a bit, you can cut and dust with more confectioners sugar or sprinkles!