Make the Cookies:

1. Preheat and Prep: Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper or silicone baking mats. Set aside.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt until well combined and no lumps remain. Set aside.
3. Cream Butter and Sugar: In a large bowl using an electric mixer (or a stand mixer fitted with the paddle attachment), beat the softened butter and granulated sugar together on medium speed for 2-3 minutes until light and fluffy. This incorporates air and creates a tender cookie.
4. Add Egg and Vanilla: Beat in the egg and vanilla extract until fully incorporated, scraping down the sides of the bowl as needed.
5. Alternate Wet and Dry: With the mixer on low speed, add the flour mixture in three additions, alternating with the buttermilk in two additions. Begin and end with the flour mixture. Mix each addition just until incorporated—don’t overmix.
6. Add Hot Water: Pour in the hot water and mix on low speed until the batter is smooth and well combined. The batter will be thick but pourable, similar to cake batter.
7. Scoop Cookies: Using a 2-tablespoon cookie scoop or measuring spoon, drop rounded portions of batter onto the prepared baking sheets, spacing them about 2 inches apart (they will spread slightly). You should get about 24 cookies total. Try to make them as uniform as possible for even baking and easy sandwich assembly.
8. Bake: Bake one sheet at a time for 11-13 minutes, or until the cookies are puffed and spring back lightly when touched. They should look set but still soft—don’t overbake or they’ll be dry. The tops won’t look shiny when done.
9. Cool: Let the cookies cool on the baking sheet for 5 minutes, then carefully transfer to a cooling rack to cool completely. This is important—the cookies must be completely cool before filling or the filling will melt!

Make the Filling:

10. Beat Butter and Shortening: In a large bowl using an electric mixer, beat the very soft butter and shortening together on medium speed for 2 minutes until smooth and creamy.
11. Add Sugar: Add the sifted powdered sugar and beat on low speed until incorporated, then increase to medium-high and beat for 2 minutes until fluffy.
12. Add Marshmallow Fluff: Add the marshmallow fluff, vanilla extract, and salt. Beat on medium-high speed for 2-3 minutes until the filling is light, fluffy, and completely smooth. It should look glossy and hold soft peaks.

Assemble:

13. Pair Cookies: Match up cookies in similar sizes so your whoopie pies look uniform.
14. Add Filling: Turn half the cookies flat-side up. Using an offset spatula, butter knife, or piping bag, add a generous amount of filling (about 2-3 tablespoons) to the flat side of each cookie. You want a nice thick layer of filling!
15. Sandwich Together: Top each filled cookie with a matching cookie, flat side down. Gently press together so the filling spreads to the edges but doesn’t squish out too much.
16. Serve: Enjoy immediately, or store for later. The flavors actually improve after sitting for a few hours as the cookies soften slightly from the filling.