Preheat the oven to 350 degrees F; line two baking sheets with parchment paper; set aside.
In a medium bowl, whisk together the flour, baking soda, and salt; set aside.
Using an electric mixer, cream together the butter, brown sugar, and granulated sugar on medium speed until light and fluffy, 2 to 3 minutes. Add the egg and vanilla extract and beat to combine, another 1 minute.
Reduce the mixer speed to low and gradually add the flour mixture, beating until the dough just comes together. Using a wooden spoon or rubber spatula, mix in the M&Ms.
Scoop 2 tablespoons of dough and roll into a ball, placing them 2 inches apart on the baking sheets.
Bake until the edges are set but the cookies are still quite pale, 12 to 14 minutes. Cool on the pan for 5 minutes, then transfer to a wire rack to finish cooling.
Mix-Ins: You can substitute or mix and match add-ins as desired. Some ideas - semisweet chocolate chips, mini M&Ms, butterscotch chips, Reese's Pieces, other candies, etc. Use seasonal M&Ms for holidays!
Storage: You can keep the cookies in an airtight container at room temperature for up to 4 days.
Freezing Dough: Place balls of cookie dough in an airtight container (place parchment between layers) and freeze for up to 3 months. Baked from frozen at 350 degrees for 12 to 14 minutes.
Freezing Cookies: Wrap in plastic wrap and store in an airtight container or resealable bag in the freezer for up to 3 months. Thaw at room temperature then enjoy!