1. Get Ready: Preheat your oven to 350°F (175°C). Line a couple of large baking sheets with parchment paper or silicone mats and set them aside.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set this aside.
3. Cream Wet Ingredients: In a stand mixer or a large bowl with a hand mixer, beat the softened butter and brown sugar together until the mixture is light and fluffy, about 2 minutes. Scrape down the sides of the bowl with a spatula as needed. Beat in the egg and vanilla extract until everything is well combined.
4. Combine Everything: With your mixer on low speed, slowly add the dry flour mixture in three additions, alternating with the buttermilk in two additions. Begin and end with the flour mixture. Mix until just combined – be careful not to overmix! Give the batter a final gentle stir with your spatula to make sure there's no flour hiding at the bottom.
5. Bake the Pies: Use a small cookie scoop (about 2 teaspoons worth) to drop batter onto your prepared baking sheets, leaving about 2 inches between each dollop. Bake for just 10 minutes, or until the centers spring back when lightly touched. Let them cool on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely.
6. Make the Peppermint Filling: While your whoopie pies are cooling, clean your stand mixer bowl. Beat the softened butter and confectioners' sugar together on medium speed until it's light and fluffy, about 2 minutes. Beat in the vanilla extract, peppermint extract, and salt. Scrape down the bowl, then add the marshmallow creme and mix until smoothly combined. Pop the filling in the fridge for at least 30 minutes to firm up a bit.
7. Assemble & Enjoy! Once the whoopie pies are fully cool and the filling is set, spread a generous amount of mint filling on the flat side of half of your whoopie pies. Top with the remaining pies to create a sandwich. Gently roll the edges of each whoopie pie in crushed candy canes for that festive crunch. For the best result and a firmer filling, chill them for another 30 minutes before serving.