In a large mixing bowl, combine butter, brown sugar, sugar, egg, yolk, vanilla extract, mint extract, and green food coloring, beating until blended.
In a medium bowl, combine flour, baking soda, and cornstarch.
Add the dry ingredients to the mixing bowl, and beat until combined.
Fold in 1 1/2 c. chocolate chips, reserving 1/2 c. for pressing into the cookies after baking.
Chill in the refrigerator for at least 2-3 hours.
Roll dough by tablespoonfuls and place onto parchment paper or silicone mat-lined baking sheets. Try to make the dough tall cylinders so they bake thick.
Bake at 325 degrees for 10-12 minutes, or until the centers are just set, and the edges are golden brown.
Remove from oven, and press 4-5 chocolate chips onto the top of each cookie.
Allow cookies to cool on the pan for 10 minutes before before removing to wire racks to cool completely.