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.
Measure your flour correctly! Adding too much flour to the recipe is the most common mistake. The best, and easiest way to measure flour is by using a scale. If you don't have one then fluff your flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it off.
Chill time is optional, in my recipe testing I did not see a major difference in the cookies either way.
Placing the M&Ms on top right after baking ensures they'll be shiny and perfect.
Your cookies will spread so give them at least two inches of space between them on the baking sheet.
Make sure the scrape the bowl down regularly when making the dough. You want the mixtures to be homogeneous throughout with no secret pools of butter lurking around.