Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, combine butter with sugar until fluffy. Add in egg and mint extract, and mix until it is smooth.
Add in the flour mixture, mix until it is fully combined. Add in the green food coloring and mix until it is fully green. Fold in the chocolate chips.
Evenly press the cookie dough into a greasted 8-inch square baking dish. Bake for 20-25 minutes.