Preheat your oven to 350℉. Line baking sheets with parchment paper or silicone baking mats.
In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
In a large bowl, beat the butter, granulated sugar, and brown sugar with an electric mixer on medium speed until light and fluffy. It may take about 2-3 minutes. Stop occasionally and scrape the sides of the bowl.
Add the vanilla extract and eggs, one at a time, beating well after each addition.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Gently fold in the Christmas M&M's and semi-sweet chocolate chips, if using.
Drop rounded tablespoons of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden but the centers are still soft. Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.
Serve and enjoy your festive M&M cookies with a glass of milk or your favorite holiday beverage
Notes
Too much flour can lead to dry, dense cookies. Spoon the flour into the measuring cup and level it off with a knife rather than scooping it directly from the bag to ensure accuracy.
If you have the time, chill the dough for at least 30 minutes before baking. Chilling helps prevent the cookies from spreading too much while baking, resulting in thicker, chewier cookies.
Ensure your butter and eggs are at room temperature before mixing.
Once you add the dry ingredients, mix the dough just until combined. Overmixing can result in tough cookies.
After removing them from the oven, let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack. This helps them set and makes them less likely to break apart while still warm.