Start by preheating the oven to 350 degrees and lining a baking sheet with parchment paper. Set it aside for later use.
Combine the butter, brown sugar, and granulated sugar until they form a creamy mixture. Add in the egg, egg yolk, vanilla extract, and peppermint mixing until well combined.
Sift in the flour, cocoa powder, and baking soda, and gently blend until the dry ingredients vanish into the mixture.
Carefully fold in the chocolate chips. Use a generous cookie scoop to create 12 equally sized cookie dough balls. Place these on the prepared baking sheet and softly press them down to about ½ inch thick.
Bake the cookies in the preheated oven for 10 minutes. Once done, let the cookies cool on the baking sheet for roughly 10 minutes before transferring them to a cooling rack.
Meanwhile, prepare the peppermint buttercream.
Using a stand mixer fitted with a whip attachment, beat together softened butter until light and fluffy. Add in the remaining buttercream ingredients and mix until combined.
Spread the cooled cookies with the peppermint buttercream frosting and sprinkle some crushed peppermint on top. Treat yourself to each delicious bite!