Stir the butter, shortening, sugar, and brown sugar together in a large bowl.
Add in the eggs and vanilla and mix until smooth.
Next add in the flour, cocoa, baking soda, and salt and mix well. Add in the mint chips and stir just until combined.
Drop the cookies by rounded teaspoons onto a greased baking sheet. Make sure they are about two inches apart because they will spread slightly as they bake.
Bake at 350° for 8 minutes. They are done when they are just set and the top has lost its gloss. At this point they may not look done but they will firm up as they cool. Let them sit on the baking sheet for 1-2 minutes before transferring to a wire rack to cool completely.
Serve these cookies warm right out of the oven. Enjoy!