Step 1: Prepare your pan. Generously grease a 9x13 inch baking dish with a neutral oil or non-stick spray. In a small bowl, whisk together the powdered sugar and cornstarch. Heavily dust the prepared pan with a generous amount of this mixture, using a fine-mesh sieve for an even coat. Tap out any excess.
Step 2: Bloom the Gelatin. In the bowl of a stand mixer fitted with the whisk attachment, sprinkle the gelatin powder evenly over 1/2 cup of the cold water. Let it sit for 5-10 minutes until the gelatin has absorbed all the water and become a thick, jelly-like paste. Do not stir.
Step 3: Prepare the Sugar Syrup. In a heavy-bottomed saucepan, combine the granulated sugar, corn syrup, the remaining 1/2 cup of water, and salt. Stir gently over medium heat until the sugar dissolves completely. Once dissolved, increase the heat to medium-high and bring the mixture to a boil. Clip a candy thermometer to the side of the pan, ensuring it doesn't touch the bottom. Boil without stirring until the syrup reaches 240°F (115°C) (soft-ball stage).
Step 4: Combine Syrup and Gelatin. Immediately remove the sugar syrup from the heat. With the stand mixer on low speed, carefully and slowly pour the hot sugar syrup in a thin, steady stream into the gelatin mixture. Increase the speed to medium-high and continue to whisk for 12-15 minutes, or until the mixture is very thick, white, glossy, and has more than tripled in volume. It should form stiff peaks when the whisk is lifted.
Step 5: Add Vanilla and Pour. Reduce the mixer speed to low and add the vanilla extract. Mix for just another 30 seconds to incorporate. Immediately pour the marshmallow mixture into your prepared pan, using a greased spatula to spread it evenly into the corners. Work quickly, as it sets fast.
Step 6: Allow to Set. Dust the top of the marshmallow layer with more of the powdered sugar/cornstarch mixture, using a sieve. Leave the pan uncovered at room temperature for at least 4-6 hours, or preferably overnight, to allow the marshmallows to fully set and firm up. Do not refrigerate.
Step 7: Cut and Coat. Once fully set, invert the marshmallow slab onto a cutting board heavily dusted with the powdered sugar/cornstarch mixture. Use a sharp, lightly greased knife or a pizza cutter to cut the marshmallow into desired shapes (e.g., 1.5-inch squares). Dip the knife frequently in hot water for cleaner cuts. Toss each marshmallow piece gently in the remaining powdered sugar/cornstarch mixture to coat all sides and prevent sticking.
Step 8: Store for Freshness. Store your freshly cut Perfect Homemade Marshmallows in an airtight container at room temperature for up to 2-3 weeks. Layers of parchment paper between them can help prevent sticking.