Preheat an oven to 325 F. Cream the butter and sugar together until light and fluffy, about 5-7 minutes. Scrape down the bowl several times to ensure a good mix.
While the butter and sugar are creaming, sift together the cocoa, all-purpose flour, and salt.
Add the eggs one at a time and beat until each one is incorporated. Add in the peppermint extract and whip until combined.
Add in the dry ingredients and beat until just combined. Divide the dough in half and place them onto a piece of plastic wrap. Wrap each unto a dish shape. Chill for an hour or freeze until firm about 30 minutes.
Dust a surface lightly with cocoa powder, roll the dough into ⅛” thickness. Cut the dough into 2” round cookies. Place the cookies onto a parchment-lined baking sheet. Freeze for 5 minutes.
Bake the cookies for 18-20 minutes until crispy. Allow them to cool most of the way on the cookie sheet before removing them to a cooling rack.
To make the buttercream, combine all the ingredients in the bowl of a stand mixer and cream the ingredients for 5-7 minutes until light and fluffy. Place the buttercream into a piping bag fitting with a round piping tip.
Once the cookies are thoroughly cooled, pipe a thin layer of frosting on the inside edge of one cookie, top with another cookie, and gently press the cookie down until the frosting reaches the edges. Repeat with the remaining cookies.
Tips
Due to the large quantity of high-quality cocoa powder in the dough, you will want to work very quickly rolling the dough and cutting it as it will become sticky fast. If this happens, stop, gather up the dough and freeze it until firm before rolling and cutting more.
Ensure the butter and eggs are at room temperature before making the dough and frosting for the best creaming.
