Mix gelatin with 1/2 cup cool water in a small bowl, then stir in vanilla extract or seeds (no need to fully incorporate the vanilla seeds, if using; they will disperse later on). Set aside.Advertisement
Combine the remaining 1/2 cup water, granulated sugar, corn syrup and salt in a medium saucepan. Cook over medium heat, stirring with a fork, until the syrup begins to simmer around the edges, 5 to 8 minutes. Clip a candy thermometer or other heatproof digital thermometer to the pot, making sure it's submerged in the liquid but not touching the bottom of the pot. Increase heat to medium-high and simmer without stirring until the syrup registers 250 degrees F, 5 to 10 minutes more.
Remove from heat and transfer the thermometer to the bowl of a stand mixer. Scrape the hot syrup into the bowl with a heatproof spatula and let cool to 212 degrees , 8 to 10 minutes.
Add the gelatin mixture to the syrup and mix on low speed with the whisk attachment for 1 minute. Increase to medium-high and whip until the fluff is roughly tripled in volume and balled up around the whisk, 8 to 10 minutes.
Lightly coat an 8-inch square baking pan with cooking spray. Scrape and smooth the fluff into the prepared pan. (If the mixture seems sticky, lightly oil or spray a spatula to help you smooth it into the pan.) Refrigerate until firm, about 1 hour. Or cover with foil and refrigerate for up to 3 days.
To cut marshmallows: Sift a bit of confectioners' sugar over the marshmallow, then invert the panful onto a cutting board. Sift more confectioners' sugar over the exposed surface and rub until no longer sticky to the touch. Cut into 36 marshmallows with a chef's knife, wiping the blade clean as needed. Put the remaining confectioners' sugar in a sealable bag and shake a few marshmallows at a time in it until the cut sides are no longer sticky.
Make Ahead Tip: Prepare through Step 5 up to 3 days ahead. Store finished marshmallows airtight at room temperature, with wax paper between layers, for up to 2 weeks.
Equipment: Candy thermometer
1 other carbohydrate