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