Line a 9x13-inch baking pan with parchment, and lightly mist it with non-stick spray.
Place ½ cup of the water in a shallow bowl and sprinkle the powdered gelatin on top, in an even layer. Set aside to soften.
Place the egg whites and salt in a large mixing bowl and whip on medium speed.
While the egg whites are whipping, heat the sugar, corn syrup, and remaining ¼ cup of water in a small pot over medium heat, swirling the pot occasionally to combine the ingredients.
When the sugar syrup reaches 245 degrees F (and the egg whites are stiff, white, and tripled in volume) remove it from the heat and drizzle it into the whites (as they continue to whip) in a thin, slow stream.
Add the softened gelatin to the pot and warm it gently over low heat until liquefied, then add it in to the mixture slowly, while continuing to whip.
Split the vanilla bean lengthwise, scrape out the seeds, and add them to the marshmallow mixture.
Transfer the mixture to the prepared pan, smoothing it into an even layer, and allow to set for about 4 hours (or overnight).
Cut the marshmallows into individual portions, and dust with cornstarch, shaking vigorously in a fine mesh sieve to remove any excess.