Combine the cornstarch and confectioners’ sugar and place in a fine-mesh sieve. Generously dust the bottom of a large rimmed baking sheet (ideally a jelly-roll or half sheet pan about 13 by 18 inches) with about half of the mixture. Set the remaining cornstarch mixture aside.

In a heavy-bottomed saucepan, combine the granulated sugar with 1 cup of the corn syrup and ¾ cup of the water. Clip a thermometer to the side of the pan, place the pan over medium-high heat, and heat, stirring occasionally, for about 15 minutes, or until the temperature registers 239°F.

Meanwhile, put the remaining ½ cup water in the bowl of a stand mixer fitted with the whip. Sprinkle the gelatin over the water and let stand, without stirring, for about 3 minutes to soften then add the remaining 1¼ cups corn syrup and beat on medium speed for about 4 minutes, or until frothy.

When the hot sugar mixture is ready, add it to the mixer bowl and beat on medium-high speed for about 5 minutes, or until medium peaks form or the mixture has reached its maximum volume. Add the chocolate and whip just until combined.

Pour the mixture into the prepared pan, lightly coat an offset spatula with nonstick vegetable spray and lightly press on the mixture to spread it evenly. Lightly and evenly dust the surface with the remaining cornstarch–confectioners’ sugar mixture by tapping on the side of the sieve. Set aside to rest at room temperature for at least 8 hours or up to overnight. Using a sharp knife, cut the sheet into 1-inch squares. Layer the marshmallows, separated by waxed paper, in an airtight container at room temperature for up to 1 week.