Preheat your oven to 350 degrees F. Line a baking sheet with parchment paper and set aside
In a medium mixing bowl, stir together the flour, baking soda, and salt. Set aside.
In a large mixing bowl, beat together butter, brown sugar, and sugar until smooth, about 1 minute. Add the egg and vanilla and beat until light and fluffy.
Add the flour mixture to the wet ingredients in 2 additions, mixing until just combined after each addition. Use a wooden spoon or rubber spatula to stir in the M&Ms by hand until evenly distributed.
Drop the cookie dough using a cookie scoop (about 1 tablespoon each) onto the prepared baking sheet. Push some extra M&Ms on top, that way they'll show when the cookies are baked.
Bake for 9-11 minutes or until the tops are no longer shiny and the edges are slightly golden. Remove from oven and allow to cool 5 minutes on the baking sheet before transferring to a wire rack to finish cooling.
Notes
The cookies will stay good in an airtight container at room temperature for up to 5 days.