For the Chocolate Whoopie Pies

1. Preheat your oven to 350 degrees Fahrenheit and line baking sheets with parchment paper. This is important for ensuring the sticky and moist whoopie pies do not stick to your pan after baking.
2. In a medium bowl, whisk dry ingredients together with a wire whisk. Stir all purpose flour, cocoa powder, baking soda, baking powder, and salt until combined.
3.  In a large bowl, whisk granulated sugar, light brown sugar, room temperature egg, vegetable oil, vanilla extract, and room temperature buttermilk. Mix until smooth.
4. Carefully, add the dry ingredients to the wet ingredients before mixing with a wire whisk until smooth. The batter should be thick. Do not overmix, as mixing past when you see the dry ingredients disappear into the batter will create dense cake pieces. Pour 2 tablespoons of hot water into the batter and mix until smooth.
5. Let the batter sit for 10 minutes at room temperature to allow the batter to gain structure and firm up.
6. Divide the batter into 1-tablespoon-sized dollops on your prepared baking sheet. The dollops should be at least 2 inches apart because they will rise and spread as they bake.
7. Bake the whoopie pies for 8-11 minutes until the tops are set and shiny and the centers spring back when quickly poked with your pointer finger.
8. Allow the whoopie pies to cool on your baking sheet for at least 15 minutes before filling.

For the Oreo Filling 

1. In a large bowl with a hand mixer or in the bowl of your stand mixer fitted with a paddle attachment, beat room temperature butter until smooth.
2. Slowly add powdered sugar to the bowl and beat on medium speed until the frosting is smooth.
3. Add marshmallow fluff to the frosting and beat on high speed until incorporated. Use a rubber spatula to scrape down the sides and the bottom of the bowl before mixing again to ensure the frosting is combined.
4. Add heavy whipping cream to your frosting and beat on high speed for at least 3 minutes. This creates a light, smooth, and velvety filling.
5. Stir Oreo cookie crumbs into the frosting. If desired, transfer the Oreo filling to a piping bag or zippable plastic bag with the corner snipped off of it to create a makeshift piping bag.

For Assembly

1. Top half of the cooled cake disks with Oreo filling. If possible, leave a small rim around the outside to prevent the frosting from spilling out.
2. Top the frosted cake pieces with an unfrosted cake piece to create a cake sandwich.
3. Store the whoopie pies in single layers or individually wrapped in cling wrap to prevent the whoopie pies from sticking to one another.