Special equipment: a 10-inch heat-safe bowl, a candy thermometer and a 9-inch springform cake pan
For the marshmallow: Line a 10-inch heat-safe bowl with a large piece of plastic wrap, leaving a 5 inch overhang. Coat the plastic wrap with cooking spray and then add a large scoop of confectioners' sugar. Pat and rotate the bowl on an angle so that the plastic wrap is coated in sugar. Tap out any excess and set aside.
Stir together the gelatin and 1/2 cup cold water in the bowl of stand mixer fitted with a whisk attachment.
Combine the granulated sugar, corn syrup, 1/2 cup water and salt in a small saucepan and cook over medium-low heat, stirring occasionally, until the sugar completely dissolves. (If any granules of sugar stick to the side of the pan, brush them back into the sugar-water mixture with a wet pastry brush.) Attach a candy thermometer to the pan and increase the heat to medium-high. Cook until the mixture reaches 240 degrees F.
With the mixer on low speed, slowly pour the sugar syrup into the softened gelatin. Once all the syrup is added, increase the speed to high and beat until the mixture is very thick and holds soft peaks, about 15 minutes. Beat in the vanilla.
Coat a rubber spatula with cooking spray and scrape the marshmallow into the prepared bowl. Smooth out the top and dust with confectioners' sugar. Let sit at room temperature until the marshmallow is completely cool and set, about 1 hour.
Meanwhile, for the crust: Preheat the oven to 375 degrees F. Coat the inside of an 9-inch springform pan with cooking spray.
Pulse the graham crackers, flour, baking powder and salt in a food processor until finely crushed. Add the butter and eggs and pulse until the mixture clumps together. Press into the prepared pan and bake until golden and no longer shiny, 15 to 20 minutes. Let cool completely. Unlock and remove the side of the pan.
For the chocolate coating: Microwave the chocolate and coconut oil together in a medium microwave-safe bowl in 30-second increments, stirring after each, until the chocolate is smooth. Let cool at room temperature until the mixture coats a spoon and drips off slowly.
Build the cookie: Line a baking sheet with a wire rack. Place the crust on the rack and pour about 1/3 cup of the chocolate coating onto the center of the crust. Using an offset spatula, spread the chocolate to the edges of the crust ensuring that the chocolate drips and coats the sides of the crust evenly. Use the sides of the plastic wrap to remove the marshmallow and invert onto the crust.
Pour the remaining chocolate coating all over the top of the cookie and let it drip to coat completely. Let sit at room temperature until the coating is hard and set, about 30 minutes.
Transfer to a cake stand or serving platter. Coat a knife with cooking spray and slice (this will help keep the marshmallow from sticking to the knife).
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