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 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. Top evenly with the green colored sugar. Bake for 20-25 minutes.