Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, baking soda, and cream of tartar. In another large bowl, combine butter, peanut butter, and both sugars until fluffy.
Add in egg and vanilla extract, and mix until smooth. Add in the flour mixture, mix until it is fully combined. Fold in the Reese's pieces.
Combine sugar and cinnamon in another shallow dish. Roll the cookie dough into balls, and roll each ball into the cinnamon sugar mixture.
Place the cookies onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.
