Chocolate Whoopie Pies with Orange-Mint Buttercream

For the cakes:

1. Preheat oven to 350F. Line a baking sheet with parchment paper.
2. In a medium bowl, mix together the flour, cocoa, baking soda and 1 teaspoon of salt until combined.
3. In a small bowl, mix together the buttermilk and vanilla.
4. In a large bowl, using a stand or hand mixer, cream together the butter and brown sugar on medium-high speed until pale and fluffy, about 5 minutes.
5. Add in the egg and beat until well combined.
6. Reduce the mixer speed to low, or hand mix at this point, and alternately mix in the flour mixture and buttermilk in batches, beginning and ending with the flour.
7. Scrape down the sides of the bowl occasionally and mix until smooth.
8. Spoon 1 tablespoon-sized scoops of batter about 1-2 inches apart onto the prepared baking sheet.
9. Bake in the upper portion of the oven for the first half of the baking time and switch to the lower third of the oven for the second half of the time.
10. Bake 10-12 minutes until the tops are puffed and the cakes spring back when touched.
11. Transfer to a rack to cool completely before assembling.

For the filling:

12. Using a stand or hand mixer, beat together the powdered sugar and butter on low until the mixture is crumbly, about 1 minute.
13. Add the heavy cream, vanilla, peppermint, salt and orange zest and beat on high until smooth, about 3 minutes.

For the assembly:

14. To assemble, spread a desired amount of filling onto the base of a cake and top with another cake.