Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, combine butter with peanut butter, granulated sugar, and brown sugar until fluffy.
Add in the egg and vanilla extract, and mix until smooth. Add in the flour mixture, and mix until combined. Add in the green food coloring, mix until fully green.
Press the cookie dough evenly into a greased 8-inch square baking dish. Bake for 20-25 minutes.