First heat the oven to 375F. Then in a large bowl stir cookie mix, butter & egg until dough forms. Then divide dough in half. Then stir cocoa into one half. Then stir flour, mint extract & food color into other half. Then place chocolate dough on 17x12 inch sheet waxed paper. Top dough with second sheet of waxed paper. Then roll dough to form 12x7 inch rectangle. Repeat with green colored dough.
Next remove top sheet of waxed paper from both doughs. Then using waxed paper to lift green dough & invert onto chocolate dough. Then gently press layered dough to 14x8 inch rectangle. Then remove top sheet waxed paper. Use bottom sheet waxed paper to help roll doughs up together tightly, beginning at long side. Then wrap tightly in waxed paper & freeze at least 2 hours or until very firm.
The unwrap the dough cut into 1/4 inch slices. Then place slices about 2 inches apart on ungreased cookie sheet. Then bake 9 to 11 mins or until set. The cool 2 mins & remove from cookie sheet to wire rack.