Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In another large bowl, combine butter, peanut butter, brown sugar, and granulated sugar until fluffy.
Add in egg and vanilla extract, and mix until smooth. Add in the flour mixture, and mix until it is fully combined. Fold in the m&ms.
Roll the cookie dough into balls, and place onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.