In a medium bowl, whisk together flour, salt and baking soda. In a large bowl using either a hand or a stand mixer, beat softened butter with both granulated sugar and brown sugar until fluffy.
Add in eggs and vanilla extract to the butter mixture. Continue to beat until smooth. In three separate additions, add in the flour mixture beating after each addition until fully incorporated.
Gently fold in M&Ms with a spatula. Cover and refrigerate dough for at least one hour. Using a cookie scoop or your hands, roll the cookie dough into small balls and place 2-inches apart onto ungreased baking sheets. Bake for 10-12 minutes.
