Preheat oven to 300*F. In a medium bowl combine flour, soda, salt and cocoa. Mix well and set aside. In a large bowl blend sugars with an electric mixer at medium speed. Add butter and beat to form a grainy paste. Scrape sides of bowl, then add eggs and mint extract. Beat at medium speed until light and fluffy. Add the flour mixture and chocolate chips, and blend at low speed just until combined. Drop dough by rounded tablespoons onto ungreased cookie sheets, 1 1/3 in apart. Bake for 19-21 minutes. Immediately transfer cookies to a cool, flat surface.