Prepare a 9 by 9 inch square pan by spraying with cooking spray. Place a piece of parchment paper in the pan to cover the bottom and two adjacent opposite sides (leaving the other two opposite sides of the pan uncovered) ; press into the creases. This makes it easy to lift out of the pan when done. Spray the parchment paper and set pan aside.
Mix together the confectioner's sugar and cornstarch in a small bowl; set aside.
In the bowl of a free standing mixer equipped with a whisk attachment, add the gelatin, Crème de Menthe and ¼ cup of the cold water. Do not stir.
In a large pan combine the sugar, corn syrup and remaining ½ cup of cold water. Stir to dissolve. Equip the pan with a candy thermometer and bring to a simmer over Medium-High heat; do not stir. Allow mixture to simmer until it reaches 240 degrees on the thermometer.
Carefully pour the sugar mixture into the bowl of the mixer with the gelatin. Start the mixer on low speed and once the two mixture have combined raise the speed to high and allow to mix for 15 minutes when mixture will be light and airy.
At 14 minutes add the peppermint extract and any food coloring if desired (the mixture will be a very pale green without any coloring).
Spray a spatula or silicone spoon with cooking spray so it won’t stick to the marshmallow. Spoon the marshmallow mixture out of the mixing bowl and into the prepared pan. Smooth with the sprayed spatula to cover the pan and smooth the top. Sprinkle some of the confectioner's sugar-cornstarch mixture on the top; reserve the rest for the next day.
Set the pan of marshmallow aside at room temperature, uncovered, overnight.
The next day sprinkle the remaining confectioner's sugar-cornstarch onto a work surface.
Lift the marshmallow from the pan with the sides of the parchment paper. Peel away the parchment paper and place the underside of the marshmallow (that was on the parchment paper) onto the sugared work surface, allowing it to become covered with the sugar-cornstarch.
Cut the marshmallow into 1 1/4 or 1 1/2 inch wide strips; kitchen scissors work the best for this task. Cut each strip into individual marshmallows (1 1/4 or 1 1/2 inch). Roll all sides of the marshmallows in the sugar-cornstarch mixture and shake off any excess (this will keep the marshmallows from sticking to each other or hands).
Store in an airtight container for up to 5 days.