STEP 1: Preheat the oven to 190ºC/375ºF and set oven racks to the upper and lower third of the oven.

STEP 2: Line 2 cookie sheets with parchment paper and set them aside. You will need to bake two or three times depending on the size of your cookie sheets.

STEP 3: In a medium bowl, combine the flour, unsweetened cocoa powder, baking soda, and salt. Stir until well combined and set aside.

STEP 4: In a large mixing bowl mix in the brown sugar, buttermilk, vegetable oil, egg, and vanilla extract of vanilla bean paste and whisk until well combined.

STEP 5: Gradually add the dry ingredients to the wet batter mixing until smooth. At this point, it should look like a thick brownie batter.

STEP 6: Add the hot water and continue to mix until smooth. The consistency should change a bit making it smooth and velvety.

STEP 7: Transfer the cookie pie batter to the lined cookie sheets with the help of an ice cream scoop or with a pastry bag (ziplock bags also work) if you feel like this will be easier. You want to drop a 1 ½ tbsp scoop of batter at a time, leaving at least 2in (5cms) in between each scoop/pie. They should be really spaced out.

STEP 8: Bake the first two sheets for 4 minutes, then rotate them (the lower cookie sheet will go to the first rack and the first rack cookie sheet will go to the lower rack) and bake for 4 additional minutes. The center of the cookies should spring back when touched.

STEP 9: Remove from the oven and allow cookie pies to cool before moving the parchment paper off the cookie sheet or attempting to pick them up. They are really soft and fluffy and will become manageable once completely cooled down.

STEP 10: Repeat step 8 and 9 until all the cookie pie batter is baked.

STEP 11: While the cookie pies cool down completely, make the filling. In a large bowl (if using a hand mixer) or in the bowl of a standing mixer, add the softened, room temperature butter and whip until creamy and fluffy.

STEP 12: Add the powdered sugar, vanilla extract, or vanilla bean paste and whisk until airy. The mix should start to whiten as you whip.

STEP 13: Once airy and fluffy, add the marshmallow fluff and fold it into the mixture. Add to a pastry bag and fill the whoopie pie making them into tiny little cookie sandwiches.