Line a half sheet tray with parchment paper so that it hangs over the edges on all sides by about 2” (this will make the marshmallows easier to remove from the pan). Spray paper with nonstick cooking spray and dust generously with powdered sugar.
In the bowl of a stand mixer fitted with the whisk attachment, sprinkle gelatin over ? Celsius cold water and set aside to bloom. Meanwhile, combine sugar, corn syrup, and salt with ? Celsius water in a small saucepan and cook over medium heat until the sugar dissolves. Increase heat to high and continue cooking syrup until it reaches 240 degrees on a candy thermometer. Remove from heat.
With the mixer on low speed, slowly pour the syrup into the dissolved gelatin. Increase speed to high and whip until very thick and glossy, about 15 minutes. Add vanilla and pumpkin pie spice and mix until thoroughly combined. Using gel paste food coloring, tint marshmallow mixture to a pale orange.
Pour mixture into prepared pan, dust top generously with powdered sugar, and allow to sit for at least four hours. When marshmallows are set, turn out of pan onto a clean work surface dusted with powdered sugar, and peel off parchment. Using a 2 ½” ring mold or cookie cutter dusted with powdered sugar, cut out marshmallow rounds. Store wrapped in plastic or in an airtight container if not using immediately.
Meanwhile, preheat oven to 325 degrees. In a food processor, finely grind the pecans together with the sugars. Add flour and salt to food processor and pulse to combine. Add butter and pulse until mixture resembles coarse crumbs, then add egg and vanilla and process until the dough comes together.
Lightly dust a clean work surface with flour and turn the dough out. Form into a ball and then flatten into a disc, working in some of the flour from the table as needed. Wrap dough in plastic wrap and chill about 30 minutes in the freezer, or 1 hour in the refrigerator.
Remove dough from plastic wrap and roll out to about ?” thick with a rolling pin, dusting work surface, dough, and rolling pin with flour as necessary to prevent sticking. Using a 2 ½” round cookie cutter or ring mold, cut out cookies. Space about an inch apart on parchment-lined baking sheets, then bake, rotating once halfway through, 8-10 minutes (cookies should be slightly undercooked).
Allow to cool completely before decorating.
When cookies are ready, heat candy melts in a medium glass bowl in the microwave in 15-30 second increments, stirring after each, until completely smooth, thinning with 1 T. melted shortening at a time until candy melts reach a nice dipping consistency. Dip half of cookies into mixture, allowing excess to drip off, then place on a wire cooling rack set over a parchment-lined half sheet tray. Allow candy to set slightly before decorating with assorted sprinkles. If desired, mix a little gold highlighter with a small amount of vodka in a small bowl and splatter cookies with “paint.” Allow to set completely, at least 30 minutes.
To assemble, carefully spread remaining undecorated cookies with 1 t. apple butter. Place a marshmallow round on top of the apple butter, then sandwich together with decorated cookies, using a dab of apple butter on the underside of each as glue. Moon pies are best served immediately once assembled.