Instructions

• Prepare an 8"x8" pan: thoroughly dust the inside with powdered sugar (use some of the 1 cup reserved for dusting). Tap out any excess and set the pan aside.
• Bloom the gelatin: pour 1/3 cup of the water into the bowl of a stand mixer. Sprinkle the 3 envelopes of gelatin evenly over the water, stir briefly to moisten, and let stand about 10 minutes until the gelatin softens.
• Make the sugar syrup: in a medium saucepan combine the remaining 1/3 cup water, 1 cup granulated sugar, 1 cup light corn syrup, and a pinch of kosher salt. Clip a candy thermometer to the side of the pan, making sure the tip does not touch the bottom. Heat over medium-high and cook without stirring until the syrup reaches 240°F (soft‑ball stage). If sugar crystals form on the sides, brush them down with a pastry brush dipped in water.
• Add the hot syrup to the gelatin: with the mixer running on low speed, very slowly pour the hot syrup into the softened gelatin in a thin, steady stream (aim between the whisk and the bowl to minimize splatter). When all the syrup is added, add 2 teaspoons pure vanilla extract.
• Whip the mixture: increase the mixer speed to medium-high and beat until the mixture turns white, becomes very thick and sticky, and begins to pull away from the sides of the bowl in string-like pieces, about 8 minutes.
• Transfer to the pan and set: using a spatula or wet hands (keep your hands damp with the remaining water to reduce sticking), spread the marshmallow mixture evenly into the prepared pan and press into the corners. Smooth the top as best you can. Let the pan sit uncovered at room temperature until the marshmallow is firm and cool, at least 1 hour.
• Cut and coat: sift some powdered sugar into a shallow bowl. Run a wet knife around the edges of the pan to loosen the marshmallow, then turn or lift the marshmallow out onto a cutting board or parchment. Sprinkle the top with powdered sugar. Using a wet knife or pizza cutter, cut into squares. Toss each marshmallow in powdered sugar until fully coated, using more of the reserved powdered sugar as needed.
• Store: keep marshmallows in an airtight container at a cool temperature in a single layer or in layers separated by wax or parchment paper. They will keep well for a couple of weeks.