Combine butter, shortening, and sugar in a stand mixer. Mix until smooth.
Add eggs and mix until fluffy.
Add salt, baking soda, cream of tartar, cocoa powder, and 1/2 cup flour. Mix until combined, then add remaining flour into 1/2 cup increments.
Once all the flour is combined add mint chips and mix until just combined.
Cover cookie dough with Saran Wrap and chill for 30-60 minutes.
While dough is chilling preheat oven to 375.
Once dough has chilled scoop dough onto cookie sheet in 1 tablespoon scoops and bake for 8-10 minutes.
Enjoy!
Notes
Since this recipe makes A LOT of cookies I like to bake half of the cookies then scoop the remaining dough onto a cookie sheet, freeze, then place in a ziplock bag to be baked later.