Cream the butter and sugars. In a large bowl, beat the softened butter with the granulated sugar and brown sugar using a hand mixer or stand mixer for 2 to 3 minutes, until light and fluffy.
Add the egg, yolk, mint, and color. Beat in the egg, then the egg yolk, then the peppermint extract. Add the green gel food coloring a drop at a time, mixing until you reach the shade of green you like.
Add the dry ingredients. In a separate bowl, whisk together the flour, cornstarch, baking soda, and salt. Add to the wet mixture and stir just until combined, then fold in the chocolate chips.
Scoop the dough. Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper. Scoop 2-tablespoon balls of dough and place them 2 inches apart on the prepared sheets.
Bake. Bake for 10 to 12 minutes, until the edges are set and the tops look just barely done. Cool on the pan for 5 minutes, then transfer to a wire rack.