Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, mix butter with peanut butter, granulated sugar, and brown sugar until fluffy.
Add in egg and vanilla extract and mix until smooth. Add in flour mixture, and mix until it is fully combined. Place each sanding sugar in a shallow dish. Roll the cookie dough into balls and roll each one in one of the sugars.
Place onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.