Step 1: Lightly grease a 9x13-inch baking dish with butter or cooking spray. In a small bowl, whisk together 1/2 cup confectioners' sugar and 1/2 cup cornstarch. Heavily dust the greased pan with this mixture, ensuring all surfaces are covered. Set aside.
Step 2: In the bowl of a stand mixer, sprinkle the 3 packets of unflavored gelatin over 1/2 cup cold water. Let it sit for 5-7 minutes to bloom and soften. Do not stir.
Step 3: In a heavy-bottomed saucepan, combine the 2 cups granulated sugar, 1/2 cup light corn syrup, and 1/2 cup water. Stir gently to combine before placing it over medium-high heat.
Step 4: Insert a candy thermometer into the syrup, ensuring it doesn't touch the bottom of the pan. Bring the mixture to a boil without stirring. Cook until the syrup reaches precisely 240°F (115°C), the soft-ball stage.
Step 5: As soon as the sugar syrup reaches temperature, immediately remove it from the heat. Attach the whisk attachment to your stand mixer and start mixing the bloomed gelatin on low speed.
Step 6: With the mixer still on low speed, carefully and slowly pour the hot sugar syrup in a thin, steady stream down the side of the bowl, avoiding the whisk. Be very careful as the syrup is extremely hot.
Step 7: Once all the syrup has been added, increase the mixer speed to medium-high (around 8 on a KitchenAid). Continue to whip for 10-15 minutes, or until the mixture is very thick, glossy, nearly tripled in volume, and forms stiff peaks. The bowl should feel warm to the touch, not hot.
Step 8: Reduce the mixer speed to low and add 1 tablespoon of vanilla extract. Mix for just 30 seconds until fully incorporated.
Step 9: Quickly, before the mixture sets too much, pour the marshmallow mixture into your prepared 9x13-inch pan. It will be very sticky.
Step 10: Lightly grease an offset spatula or your hands with cooking spray or butter. Gently spread the marshmallow evenly into the pan.
Step 11: Dust the top of the marshmallow with an additional generous layer of the confectioners' sugar/cornstarch mixture. Leave the pan uncovered at room temperature for at least 4 hours, or preferably overnight, until firm and no longer sticky.
Step 12: Once set, invert the marshmallow onto a clean cutting board heavily dusted with the confectioners' sugar/cornstarch mixture. Using a sharp, lightly oiled knife or a pizza cutter, cut the marshmallow into desired shapes and sizes (e.g., 1-inch squares).
Step 13: Toss the cut marshmallows gently in the remaining confectioners' sugar/cornstarch mixture, ensuring all sticky sides are coated. Shake off any excess. Store in an airtight container at room temperature for up to 2-3 weeks.