Preheat the oven to 350 F. Line a muffin tin with 12 muffin liners. In a medium bowl, whisk together flour, salt, and baking soda.
In another large bowl, combine butter, peanut butter, brown sugar, and granulated sugar, and mix until fluffy. Add in egg and vanilla extract, mix until smooth.
Add in the flour mixture, and mix until a thick dough forms. Fold in m&ms. Form the mixture into balls, and place each ball into a muffin tin. Bake for 15-20 minutes.