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, and sugar until fluffy.
Add in egg and vanilla extract, mix until smooth. Add in flour mixture, mix until a thick dough forms. Roll the cookie dough into balls.
Place at least 2-inches apart onto ungreased baking sheets. Bake for 10-12 minutes.
Remove from oven, and decorate with pretzels, eyes, and m&ms.