Grease a 9x9-inch baking pan with cooking spray and set aside.
In the bowl of a stand mixer, add ⅔ cup cold water. Sprinkle gelatin on top and let bloom.
In a medium saucepan over medium-high heat, combine granulated sugar, corn syrup and ⅔ cup water. Stir until sugar is dissolved. Bring the mixture to a boil, swirling the pan occasionally, until the mixture reaches exactly 240°F on a candy thermometer, about 10-12 minutes. Immediately remove from heat.
With the stand mixer on low speed, slowly and carefully pour the melted sugar mixture into the gelatin mixture. Add the vanilla extract, vanilla bean paste (optional) and salt.
Slowly (to avoid splashes) increase the speed to high and whip until the mixture is stiff and white, about 10-12 minutes. When you remove the whisk, stiff peaks should form that take a few seconds until they fall back into the marshmallow mixture. Working quickly, pour the mixture into the prepared baking pan and use a spatula to help spread evenly.
Let the marshmallows sit and firm for at least 4 hours at room temperature or 2 hours in the refrigerator.
Lightly spray your work surface with cooking spray. Use a butter knife to slice around the edges of the pan, then turn the marshmallow out onto the work surface. You may need to pry it out at a corner. Use a sharp knife to cut the marshmallow into 16 pieces. If needed, spray the knife with cooking spray to help prevent sticking. If there is excess cooking spray on your marshmallow, dab with a paper towel to remove.
Roll each marshmallow in powdered sugar or cornstarch on each side to prevent sticking, then serve.
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Notes
To make these marshmallows without corn syrup, you can use agave nectar, honey, or maple syrup in a 1:1 substitute for the corn syrup. Keep in mind, you will taste those agave nectar, maple syrup, or honey flavors in the final marshmallow.Gelatin substitutes: There is no easy substitute for gelatin to make these marshmallows vegan. I had previously recommended agar agar, but it is not as simple as making a 1:1 substitution.