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, mix until smooth. Add in the flour mixture, mix until a thick dough forms. Fold in the m&ms. Place powdered sugar into a shallow dish.
Roll the cookie dough into balls, and roll each one into the powdered sugar, coating it fully. Place the cookie dough onto a large ungreased baking sheet, at least 2-inches apart. Bake for 10-12 minutes.
