Spray a 9"x13" pan lightly with unflavored oil to prevent sticking.
Add ½ cup cold water and add vanilla extract and vanilla bean paste to a stand mixer or large bowl. (I like to do ½ Tbsp vanilla extract, and ½ Tbsp vanilla bean paste, but you can use 1 Tbsp vanilla extract for a classic light vanilla flavor. Or scrape the seeds of a vanilla bean into the mixture instead of using the vanilla bean paste.)
Add 2 Tbsp (3 envelopes, approx 21g) gelatin into the bowl as you whisk lightly to incoporate it so it doesn't clump.
In a heavy saucepan, add remaining ½ cup cold water, 2 cups granulated sugar, ½ cup corn syrup, ¼ tsp salt. Cook over low heat, stirring gently until sugar dissolved.
Increase heat to medium and boil without stirring until mixture reaches 240°F, about 12 minutes. As soon as it reaches the right temperature remove from the heat.
Begin whisking the bloomed gelatin on slow speed. Pour the sugar mixture over the gelatin bloom stirring until dissolved evenly. Then add the remaining sugar mixture.
Beat mixture on high speed until white, thick, and nearly tripled in volume. This should take approximately 6-8 minutes in a stand mixer, and roughly 10 minutes with a hand mixer.
Working quickly, spread mixture into the prepared baking pan and smooth with an oiled spatula.
Allow to stand, uncovered for at least 4 hours up to one day.
Mix together a 50-50 mixture of cornstarch and powdered sugar. Turn out the marshmallows and using a pastry brush, coat with the sugar mixture. Cut into desired sizes, I like to trim the edges, and then cut into 5 columns, Cut each column into squares, makes roughly 25. Brush each individual marshmallow with the sugar mixture on all sides to prevent sticking.