Spray an 8x8” pan with cooking spray.
Make the coating mixture by whisking powdered sugar and cornstarch in a small bowl and set aside.
Whisk the gelatin and 1/2 cold water in a small bowl and let sit for 5 minutes to soften.
Stir the sugar, 1/4 cup corn syrup, 1/4 water, and salt in a medium saucepan over high heat. Boil, stirring occasionally, until the temperature reaches 240 degrees on a candy thermometer. Meanwhile, pour the remaining 1/4 cup corn syrup in the bowl of an electric mixer fitted with a whisk attachment. Microwave the gelatin mixture until completely melted, about 30 seconds. Add to the mixer bowl and let it run on low.
When the syrup reaches 240 degrees, slowly pour it into the mixer bowl. Turn mixer to medium (about level 4) and beat for 5 minutes. Increase to medium-high (level 6-8) and beat for 5 more minutes. Beat on the highest setting for 1-2 minutes and then turn to low and add vanilla and butter emulsion. (The mallow will have tripled in volume.) Turn off mixer and stir in sprinkles.
Pour marshmallow into prepared pan and sift about 2/3 of the coating mixture evenly over the top. Let sit for at least 6 hours to set.
Use a knife to loosen the marshmallow from the pan. Invert onto a cutting board sprinkled with the remaining coating mixture. Cut into squares, dipping the edges in the coating mixture as needed and pat off the excess.