Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking powder. In a large bowl, combine butter with cream cheese and sugar until fluffy. Add in egg and mint extract until 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. Evenly spread the cookie dough into a greased 8-inch square baking dish. Bake for 20-25 minutes.