Spray a 9x13-inch glass or ceramic baking pan with cooking spray. Mix powdered sugar and cornstarch, then dust the pan generously, reserving some.

In a stand mixer bowl, add gelatin and "⅔ cup cold water". Mix for 30 seconds, then let bloom for 20 minutes.

In a saucepan, whisk sugar, salt, and remaining 1 cup water. Bring to a boil and cook until 240°F (about 15 minutes).

With the mixer on low, slowly pour the hot sugar mixture into the gelatin. Increase speed to medium-high and beat for 10 minutes until thick and fluffy.

Beat in egg whites for 5 minutes, then mix in vanilla extract and vanilla bean seeds.

Pour marshmallow into the prepared pan and smooth. Let sit uncovered at room temperature for at least 8 hours.

Once firm, prepare a parchment-lined surface dusted with the reserved cornstarch/sugar mixture.

Use a sharp knife to loosen edges, then turn the marshmallow out onto the prepared surface.