Preheat the oven to 350°F (180°C) and line a baking sheet with parchment paper.
For the cookies, in a large bowl, cream together butter and sugars, then add and mix in the egg.
To the same bowl, sift in your flour, cocoa powder, baking soda, and salt. Mix until thoroughly combined.
Use a kitchen scale to weigh out 15 grams of cookie dough per cookie. Roll into a ball and place on your prepared baking sheet, keeping 1.5 inches of distance between each cookie to allow them to spread. Do not flatten the cookies.
Bake in the preheated oven for 8-10 minutes. The longer you cook them, the crunchier the cookies will be. For softer cookies, bake for 8 minutes. For crunchier cookies, bake closer to 10 minutes.
While the cookies are baking, make your icing by beating with a hand mixer the butter, marshmallow creme, salt, and vanilla.
Add in your powdered sugar and carefully mix until the sugar is combined and the icing is smooth.
Fill a piping or storage bag with your icing and cut the tip off to make a medium sized hole. You don’t want it too small, but the size of the hole in the bag doesn’t matter much as you’re just looking for an easy way to fill the Oreo cookies. You could even spread it on with a knife (though a bag is much easier).
Bake your cookies in batches and allow them to cool completely before icing.
When ready to ice, flip your cookie upside down so the flat side is getting the icing. Pipe or spread as much icing as you’d like (I wound up with some icing left over when I iced mine, so you can even double stuff some if you like) and top the icing with another cookie, flat side down.
Finish icing and assembling all your cookies and enjoy by themselves or with a glass of milk!