Preheat the oven to 350°F (175 °C). Line a couple of baking sheets with parchment paper and set aside for later.
Sift together the flour, cocoa powder, baking soda, baking powder and salt into a medium bowl. Set aside.
In the bowl of an electric mixer fitted with a paddle attachment, cream together the butter and the sugar.
Add the eggs, one at a time, along with the 1 teaspoon vanilla, scraping down the sides when necessary and beating well between additions.
Add the sifted flour mixture alternately with the buttermilk, scraping down the sides of the bowl. Continue beating just until mixed (do not overbeat).
Dollop heaping tablespoonfuls on the prepared pans, smoothing the tops and sides with your spoon and being sure to space them about 1.5 inches apart.
Bake for about 10 minutes (you can check that they are done by inserting a toothpick into the center of one—make sure it comes out clean).
Let chocolate cookies cool for a minute before transferring them to a wire rack to cool completely.
When the cookies are cool, prepare the filling by beating together the butter with the icing sugar and 1.5 teaspoon vanilla, in the bowl of an electric mixer fitted with the paddle attachment. Beat until the filling is light and fluffy, about 3 minutes.
To assemble the whoopie pies, spread a heaping tablespoon of the filling on the flat side of one cookie, and top with another. Roll the edges in crushed candy cane. Whoopie pies are best enjoyed the day they are made.
Notes
For the cocoa powder, I used Cacao Barry Extra Brute, a cocoa powder available on , I used Amazon