Step 1: Preheat and Prepare

Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a non-stick silicone baking mat (Silpat). This ensures your cookies bake evenly without sticking.

Step 2: Cream the Butter and Sugars

In a medium bowl, use a spatula or a whisk to cream together the butter, brown sugar, and granulated sugar. This step is crucial for creating that soft and chewy texture, as the sugar and butter blend into a smooth base.

Step 3: Incorporate Wet Ingredients

Add the egg yolk and vanilla extract to the creamed mixture. Mix until the ingredients are smooth and fully incorporated. The egg yolk adds richness, while the vanilla enhances the flavor profile of the cookies.

Step 4: Add Dry Ingredients

Sprinkle the flour, corn starch, baking soda, and salt over the wet ingredients. Gently mix just until combined. Avoid overmixing, as it can lead to tough cookies. The corn starch plays a key role in keeping the cookies soft and tender.

Step 5: Fold in the Chocolate and M&M’s

Gently fold in the semi-sweet chocolate chips and mini Christmas M&M’s, ensuring they are evenly distributed throughout the dough. The mix of colors from the M&M’s will add a festive touch to every bite.

Step 6: Shape the Cookies

Using a medium-sized cookie scoop, portion out the dough into 10 equal balls. Place them on the prepared baking sheet, leaving enough space between each cookie to allow for spreading. Press a few extra M&M’s and chocolate chips on top of each dough ball for an eye-catching finish.

Step 7: Bake to Perfection

Bake in the preheated oven for 8–9 minutes. The cookies will appear slightly underdone and gooey in the middle when you pull them out—this is exactly what you want! They will continue to cook and firm up as they cool.

Step 8: Cool and Serve

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a cooling rack. This step helps them set and ensures the perfect texture. Once cooled, enjoy these delightful treats with your favorite holiday beverage.