If you’re using gelatin sheets, fill a medium bowl with ice water and submerge the sheets in the water. If you’re using powdered gelatin, pour the 1/3 cup (2.9 oz / 80 g) of water into a small bowl and sprinkle the gelatin evenly over the surface. Let either stand for 5 to 10 minutes. Meanwhile, sift the cornstarch and confectioners’ sugar into a small bowl.
Line an 8 by 8-inch baking pan with 2 sheets of parchment or waxed paper, laying the sheets perpendicular to each other so that the bottom and all sides of the pan are covered. Sift enough cornstarch mixture into the prepared pan to completely and generously cover the bottom. Reserve the remaining cornstarch mixture.
In a small heavy saucepan over medium-high heat, stir together 1 tablespoon of moonshine, the granulated sugar, agave nectar, 2 tablepoons of water, and salt. Bring the mixture to a boil without stirring and cook until the temperature registers 238°F to 240°F on a digital thermometer.
Meanwhile, if using gelatin sheets, lift the softened sheets out of the bowl, squeeze out the excess water, and put the gelatin and the remaining 2 tablespoons of the moonshine in the bowl of a stand mixer fitted with the whisk attachment. If using powdered gelatin, add the bloomed gelatin to the mixer bowl along with 2 tablespoons of the moonshine.
With the mixer turned off, pour all of the hot sugar syrup over the gelatin. Whip on low speed for 30 seconds, increase the medium speed and beat for 30 seconds, and then increase the speed to high and whip for 10 to 11 minutes until the mixture is smooth, glossy, and holds medium-firm peaks. It won’t begin to resemble marshmallow until around the 5-minute mark.
Working quickly, transfer the mixture to the prepared pan and smooth the surface with a greased offset spatula. Let stand at room temperature for 3 to 4 hours until set.
Sift a generous amount of the reserved cornstarch mixture over the surface of the marshmallow. Remove from the pan by running a knife along any stuck edges, and then carefully peel away the parchment paper. Cover all of the edges with the cornstarch mixture. Using a knife or scissors dusted with the cornstarch mixture, cut the marshmallow into 2-inch squares. Generously dust all cut edges with cornstarch mixture to prevent sticking.