Beat the butter and sugars with an electric mixer on medium speed until light and fluffy, about 3-5 minutes. Add the eggs and vanilla and blend until combined, scraping down the sides of the bowl if necessary.
Combine the flour, salt, and baking soda in a separate mixing bowl and add to the butter mixture, mixing on low, one cup at a time. Stir 1½ cups of the M&Ms in by hand.
Cover and chill the dough for one hour. Towards the end of the chilling time, preheat the oven to 350 with the rack in the center. Prepare 2 cookie sheets with parchment paper or cooking spray.
Drop the dough by tablespoon-sized balls onto the parchment-lined baking sheets. Space them 2 inches apart, as the cookies will spread. Press 3 additional M&Ms on top of the dough balls.
Bake at 350° for 11-13 minutes. Cool on wire rack.
You don't have to press M&Ms on top, but it makes the cookies look much nicer. Choose one of each color for the most festive look.