Preheat oven to 400 degrees. Combine cake mix, eggs, water, cooking oil, and cocoa. You will need to blend this together well; this will be a very sticky mess. Let stand for 20 minutes, and then shape into very small 1/2 inch balls. Place these balls on a cookie sheet 2 inches a part, and smash down flat. You will need to spray a large spoon with Pam to make them flat. Bake for about 8 minutes.
Let cool until they reach room temperature. Heat chocolate chips in either the microwave or in a double boiler. When completely melted add a couple drops of the mint flavoring. Be careful not to add too much, it is a powerful flavoring. You can either spread the melted chocolate on the cookies, or you can dip the cookies into the chocolate.
Or you can buy you a bag of Keebler's Grasshopper Cookies. This taste is close; you might want to experiment by adding a little more cocoa to the cookies. Oh yes, Duncan Hines is my pick on cake mixes, just like Ghirardelli Double Chocolate Chips, actually, any of the Semi-Sweet Chips will be fine :-)