• Combine powdered sugar and cornstarch. Spray a 9x13 inch pan with non stick spray. Add powdered sugar mixture and tap and rotate pan until it is completely covered. Tap out any excess powdered sugar mixture and set aside.

To make marshmallows:

• Place ½ cup of the cold water in the bowl of stand mixer. Sprinkle gelatin over water and stir until evenly moistened. Let it sit, and as it sits it will firm up a bit, and that's okay. (During this time, the gelatin absorbs the water, this is called "blooming").
• Combine remaining ½ cup water in a saucepan with granulated sugar, corn syrup, and salt. Set over medium heat and bring to a boil, stirring to dissolve sugar.
• Once mixture comes to a boil, insert a candy thermometer and do not stir anymore. Cook sugar mixture to 240°F. (It will come to 220°F pretty quickly, and then take about 5-8 minutes to increase to 240°F. Keep a close eye, still no stirring.)
• As soon as sugar reaches 240°F, remove from heat and pour into the stand mixer bowl over the gelatin.
• Use a whisk attachment Once all the hot sugar has been added, increase speed to medium high and beat until mixture is white and thick and bowl is no longer hot to the touch. This will take about 5 minutes but will depend on exact speed and power of mixer. Add vanilla during the last few minutes of mixing, once marshmallow starts to thicken.
• How to know if marshmallow is done: Stop mixer, lift whisk and watch how marshmallow mixture falls back into bowl. It should hold its shape for a few moments and then absorb back into the rest of the mixture. (If marshmallow flows quickly off whisk and immediately absorbs back in then you need to beat longer, if marshmallow is stiff and doesn't flow off whisk without scraping then it is over beaten).
• Immediately scrape mixture out of bowl with a spatula into prepared pan. Use spatula to gently spread it into an even layer.
• Let marshmallows dry uncovered for at least 4 hours before slicing.

To cut marshmallows:

• Sprinkle more powdered sugar mixture over marshmallows and lift out of pan. Place on a cutting board and cut with pizza wheel or sharp knife as desired. If marshmallows get sticky while cutting, add more powdered sugar mix.
• Toss cut marshmallows with more powdered sugar mix, making more as needed. Marshmallows should be coated well and not stick together.
• Store marshmallows in an airtight container for several weeks.