Preheat the oven to 350° and line a baking sheet with parchment paper.
In a medium bowl, mix together the flour, baking soda and salt and set it aside.
In another large bowl, beat the butter and both sugars until creamy. Add the eggs to the butter and sugar mixture, one at a time, thoroughly mixing before adding the next. Add the vanilla and peppermint, then beat in the cocoa powder until combined. Add the dry ingredients and stir until you have nice and thick dough. Mix in the chocolate chips—making sure that the chips are spread evenly through all of the dough.
Use a ¼-cup measuring cup or an ice cream scoop to drop cookies about 1 inch apart on the baking sheet and bake for about 14 to 16 minutes. Let them cool for around 2 minutes then transfer the cookies to a cooling rack and let them cool completely before eating.