Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, combine butter and sugar until fluffy. Add in the egg and peppermint extract until smooth.
Add in the flour mixture, and mix until the flour is fully combined. Fold in white chocolate chips and crushed candy canes. Press the cookie dough into a greased 8-inch square baking dish. Bake for 15-20 minutes.