Preheat oven to 360F. Combine flour, salt, and baking soda in a large bowl then whisk together to combine and set aside.
Cream the room temperature butter using a stand mixer fitted with a paddle attachment or an electric hand mixer. Add the granulated and brown sugars and beat on hight until light and fluffy scraping the bowl down as needed.
Mix in the egg and vanilla then scrape the bowl down and add the flour mixture. Mix until just combined then use a spatula to fold in the M&Ms.
Portion into into 2 tablespoon-sized pieces and roll into balls. Place cookies on a parchment or silicone-lined baking sheet about two inches apart then bake for about 9 minutes or until the edge is a light golden color.
As soon as the cookies come out of the oven gently press more M&M's onto the tops and allow to cool completely on the baking sheet.
