Cream butter and sugar. Beat the softened butter, granulated sugar, and brown sugar with an electric mixer on medium speed until light and fluffy, about 3-5 minutes. Then, add the eggs and vanilla extract and blend until combined, scraping down the sides of the bowl as needed.
Mix dry ingredients. Whisk together the all-purpose flour, baking soda, and salt in a separate mixing bowl.
Combine. Gradually add the dry ingredients to the butter mixture, mixing at low speed, one cup at a time, until combined. Then, stir in 1½ cups of the Halloween-themed M&Ms by hand.
Chill. Cover the dough and chill it in the refrigerator for 1 hour.
Prep. Towards the end of the chilling time, preheat the oven to 350°F (175°C) and position the oven rack in the center. Prepare two baking sheets with parchment paper or lightly coat them with cooking spray.
Shape. Drop the dough by tablespoon-sized balls onto the prepared baking sheets, spacing them about 2 inches apart. Press 3 additional M&Ms on top of each dough ball for decoration (try to get one of each color).
Bake. Bake at 350°F (175°C) for 11-13 minutes or until the edges are lightly golden. Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire cooling rack to cool completely.
Notes
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 best look.