- Cream the butter and sugar.   In a large mixing bowl, beat ¾ cup softened butter, ¾ cup granulated sugar, and ¾ cup packed brown sugar on medium speed until smooth.  Then, add the eggs and vanilla and continue beating for 3-5 minutes until the mixture is light and fluffy.  Next, scrape down the sides of the bowl as needed to make sure it's even mixing. 
- Mix the dry ingredients:  Whisk together 2¼ cups plus 2 tablespoons of all-purpose flour, 1 teaspoon baking soda, and ¾ teaspoon salt in a separate bowl.  Slowly add the dry ingredients to the butter mixture, mixing at low speed, one cup at a time, until combined. 
- Add the M&Ms.  Using a spatula or wooden spoon, fold in 1½ cups of Christmas M&Ms by hand until evenly distributed throughout the dough. 
- Chill the dough.  Cover the mixing bowl with plastic wrap and chill the dough in the refrigerator for at least 1 hour.  Chilling the dough helps the cookies keep their shape and enhances their flavor. 
- Preheat the oven and prepare the baking sheets.  Toward the end of the chilling time, preheat your oven to 350°F (175°C) with the oven rack in the center.  Then, line two baking sheets with parchment paper or lightly coat them with cooking spray. 
- Shape and top the cookies.  Use a tablespoon or a small cookie scoop to portion the dough into balls, placing them about 2 inches apart on the prepared baking sheets.  Press 2-3 additional M&Ms on each dough ball for a festive look. 
- Bake.   Bake in the oven for 11-13 minutes or until the edges are lightly golden.  The centers may still look soft with a chewy texture as they cool. 
- Cool.  Let the cookies cool on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely.