Line a 12×17-inch (at least 2 inches tall) baking dish with a non-stick silicone mat; spray generously with non-stick spray.
Bloom gelatin: whisk gelatin with cold water in a small bowl. After 5–10 minutes, dice the firm gelatin into medium cubes. Set aside. Scrape vanilla bean seeds and set aside.
In a medium pot, combine granulated sugar, corn syrup, and 1 cup water. Bring to a boil over high heat, stirring to dissolve sugar. Cook to 260°F on a candy thermometer, about 10 minutes.
Once the syrup reaches 250°F, start whipping egg whites in a stand mixer on medium speed until frothy.
When syrup reaches 260°F, remove from heat. Reduce mixer to low and slowly pour syrup in a thin stream into egg whites, being careful not to splatter. Once added, increase speed to high until the mixture doubles in volume and is glossy and still warm.
Reduce to low. Add gelatin cubes a few at a time. Add vanilla extract and vanilla bean seeds. Keep on low until gelatin has melted, 1–2 minutes. Increase to high and beat until very stiff and tripled in volume, 4–6 minutes.
Working quickly once the bottom of the bowl is just slightly warm, pour into prepared pan. Spread with an offset spatula. Allow to set at room temperature for about 2 hours.
Whisk together confectioners sugar and remaining cornstarch. Cut marshmallows into 1-inch cubes and toss in the sugar mixture to coat. Store in an airtight container up to 4 days at room temperature or 2 weeks refrigerated.
