In a large bowl, combine powdered sugar and cornstarch; set aside. Line a 9 inches x 13 inches glass baking dish with foil. Lightly coat foil with non-stick cooking spray; set aside. Pour 3/4 cup cold water into a large bowl of an electric stand mixer. Then sprinkle gelatin over water and let soften. Fit mixer with a whisk attachment.
In a medium, heavy, deep saucepan, add sugar, corn syrup, water, and salt; stir to combine. Place over medium-high heat until sugar dissolves, about 3-4 minutes, stirring occasionally. Attach the candy thermometer to the saucepan and continue to cook until mixture reaches 240 degrees F, about 10-12 minutes; do not stir. Remove from heat.
Turn mixer on low speed. Slowly pour a stream of sugar syrup down the side of the bowl into softened gelatin. After all the sugar syrup has been added, gradually increase the speed of the mixer to high and beat until mixture is thick about 15 minutes. Add Nielsen Massey vanilla bean paste during the last minute of beating.
Pour marshmallow into prepared baking dish and spread evenly with a spatula; place in refrigerator until firm, about 3 hours. Sift half of the powdered sugar-cornstarch mixture onto a piece of parchment paper. Turn Vanilla Bean Marshmallow onto sugared surface and remove foil. Using a pizza wheel cutter, cut into 1 inch squares. Roll marshmallows in remaining powdered sugar mixture. Store in an air-tight container.