In a large bowl with a hand mixer or the bowl of a stand mixer, cream butter, brown sugar, and sugar until light and fluffy. Add egg and vanilla and mix until combined.
In a separate bowl, combine the flour, cocoa powder, baking soda and salt. Gradually add to the butter mixture and mix until almost combined. Add the chocolate chips, and peppermint bark chips and mix until combined. Cover and chill cookie dough for 2-4 hours or overnight.
Remove cookie dough from the refrigerator 10 minutes before baking (30 minutes if refrigerated overnight) to make cookie dough easier to scoop. Preheat oven to 350 and line a large cookie sheet with parchment paper or a silicone baking mat.
Using a medium cookie scoop, drop rounded balls of cookie dough baking sheet. Bake for 9-10 minutes, Cookies will be mostly set. Press a couple small pieces of peppermint bark into the tops of he cookies before they cool.
Let cookies cool for 5 minutes on the cookie sheet before removing to a cooling rack to cool completely.
Notes
This cookie dough is pretty soft, so don’t skip on chilling the dough.