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