In a medium bowl, sift together the flour, baking powder, salt, baking soda, and cocoa powder, set aside. Line a baking sheet with a silicone baking mat or with parchment paper, and set aside.
Mix the egg, sugar, and vanilla together in a stand mixer or with a hand mixer. Add the sour cream, ½ cup at a time, until well combined.
Once the wet ingredients are well combined, add half of the dry ingredients. Continue adding the dry ingredients by spoonfuls until all of the dry ingredients are combined, stopping a couple of times to scrape down the sides of the bowl to incorporate all of the ingredients.
Using an ice cream scoop, drop the cookie dough by scoops onto the prepared baking sheet. Bake, 12-14 minutes, or until the cookies are baked through.
Remove from the oven and allow to cool completely on a cooling rack.
Once cookies are completely cool, slather the bottom (the flat side) of one cookie with marshmallow fluff. Dip the marshmallow fluff into the crushed peppermint candies, and top it with a second cookie.
This recipe makes six large moonpies using a large ice cream scoop. If you're making smaller cookies, about 15 moonpies, you’ll want to reduce the baking time to 8-10 minutes. Mine used a cookie dough scoop that’s about 2 tablespoons, and I made about 9 moonpies (or 18 cookies), and they took 12 minutes to bake.