Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, mix butter with 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. Fold in the jello mix. Press the cookie dough into a greased square 8-inch baking dish. Bake for 20-25 minutes.