In a small bowl, combine powdered sugar and cornstarch. Set aside. Line a 9x13 pan with foil, spray with non-stick cooking spray, and coat the bottom and sides with some of the powdered sugar mixture. Pour excess powder back into the bowl. Set aside the pan.

In your stand mixer, add gelatin and 1/2 c. water. Stir to combine. Attach whisk attachment and have vanilla ready.

In a medium sauce pan, add sugar, corn syrup, salt, and 1/2 c. water. Stir in a bit until combined. Place on medium high heat, cover, and heat for 4 minutes. Remove cover and clip on a candy thermometer. Turn heat up a little more, cooking until thermometer reads 240 degrees. Quickly remove from heat.

Turn mixer on low and begin to pour syrup down the side of the inside of the bowl. Once all of the syrup is in the mixer, whisk on high. Add vanilla. Continue to whip until mixture becomes very thick and the bowl is lukewarm (roughly 10-14 minutes).

Pour mixture into prepared pan. Using a slightly oiled spatula, spread until fairly even. Sprinkle remaining powdered sugar mix on top. Allow at least 4 hours or overnight to set (uncovered).

Lift the marshmallow slab out of the foil and onto a cutting board that's sprinkled with powdered sugar. Cut into 1 inch cubes and roll in powdered sugar.

Keeps 1-2 weeks in an air tight container. Makes wonderful homemade Christmas gifts.