In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy.
Beat in the eggs one at a time. Add pure peppermint extract and a few drops of green food coloring (if desired) for that signature minty hue.
In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, and salt.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Gently fold in the semi-sweet chocolate chips and white chocolate chips. The more chocolate, the better!
Spread the cookie dough evenly into a greased 9x13-inch baking pan.
Bake in a preheated oven at 350°F (175°C) for 25-30 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.