- Line a 9×13 inch pan with plastic wrap and spray it lightly and uniformly with cooking spray. Set aside. Pour the 1/2 cup cold water into the bowl of a stand mixer and sprinkle the gelatin on top; set aside for 10 minutes.
- Combine the white sugar, corn syrup, and 1/4 cup water in a saucepan. Bring to a boil over medium-high heat. Do not stir once the sugar syrup begins to boil. Heat to 250 degrees F (120 degrees C), or until a small amount of syrup dropped into cold water forms a rigid ball. Meanwhile, combine the pumpkin puree, cinnamon, ginger, nutmeg, and allspice. Mix well.
- Attach the mixing bowl to the stand mixer with the wire whip attachment and start the mixer on low speed. Carefully pour the boiling sugar syrup in a steady stream down the side of the mixing bowl. Increase the mixer to medium-high speed and add the salt. Mix until stiff, shiny, and white in color, about 5 to 12 minutes. Scrape down the sides of the bowl occasionally using a rubber spatula.
- Remove the bowl from the mixer and fold in the pumpkin mixture until combined. Add food coloring, if desired, for a deeper orange color. Return the bowl to the mixer and beat on medium-high speed about 3 minutes longer. Dip a spatula into the marshmallow mixture: the mixture should form a more-or-less continuous ribbon off the spatula into the bowl.
- Pour the mixture into the prepared pan so that it is an even depth. Let the mixture set at room temperature for at least 8 hours or overnight.
- Sift the cocoa powder, cornstarch, and confectioners’ sugar into a pie plate. When the marshmallows have set, turn them onto a work surface and cut them into squares or shapes as desired. Roll each marshmallow in the cocoa mixture to coat all sides (reserve the cocoa mixture for re-dredging if needed). Marshmallows are best served fresh, but they can be stored in an airtight container (unrefrigerated) for about a week or in the freezer for a month. If you store the marshmallows, re-roll them in the cocoa mixture before serving.