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