Step 1: Cream the Butter and Sugars

In a large mixing bowl, use an electric mixer to beat the softened butter, granulated sugar, and brown sugar until light and fluffy, about 2–3 minutes. This step incorporates air into the mixture, ensuring the cookies are soft and tender.

Step 2: Add the Egg and Vanilla

Mix in the egg and vanilla extract until well combined. Scrape down the sides of the bowl as needed to ensure an even mixture.

Step 3: Combine Dry Ingredients

In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix, as this can make the cookies dense.

Step 4: Fold in the M&Ms and Chocolate Chips

Using a spatula or wooden spoon, gently fold in the M&Ms and chocolate chips (if using). Reserve a small handful of M&Ms to press onto the tops of the cookies before baking for a colorful presentation.

Step 5: Chill the Dough

Cover the dough with plastic wrap and refrigerate for at least 1 hour. Chilling the dough helps prevent the cookies from spreading too much while baking and enhances their flavor.

Step 6: Preheat the Oven

Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper or silicone baking mats.

Step 7: Shape the Cookies

Scoop the chilled dough into 1.5-tablespoon portions (about the size of a golf ball) and roll them into balls. Place them on the prepared baking sheets, spacing them about 2 inches apart. Gently press a few reserved M&Ms onto the tops of each cookie for a vibrant look.

Step 8: Bake

Bake the cookies in the preheated oven for 10–12 minutes, or until the edges are lightly golden but the centers still look slightly underbaked. This ensures the cookies remain soft after cooling.

Step 9: Cool and Serve

Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. Serve warm with a glass of milk or store them in an airtight container for up to a week.