1-First Step: Preparation and Mise en Place Before you start mixing, it is vital to prepare your workspace and ingredients. Take the butter out of the fridge at least an hour before baking to ensure it softens to room temperature. This step is crucial because cold butter does not cream well with sugar, leading to dense cookies. Crack your eggs into a small bowl to check for freshness and remove any shell fragments. Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius) and line two large baking sheets with parchment paper or silicone baking mats. This preparation prevents sticking and makes cleanup much easier.

2-Second Step: Creaming the Butter and Sugars In a large bowl or the bowl of a stand mixer fitted with a paddle attachment, combine the softened butter, granulated sugar, and packed brown sugar. Beat them on medium speed for about 3 to 5 minutes. You want the mixture to become pale in color and fluffy in texture. This process incorporates air bubbles into the dough, which helps the cookies rise. Do not rush this step, as proper creaming is the secret to a texture that is crisp on the edges but soft in the center. Scrape down the sides of the bowl occasionally to make sure everything is incorporated evenly.

3-Third Step: Adding Wet Ingredients Reduce the mixer speed to low and add the eggs one at a time, mixing well after each addition. Add the vanilla extract with the second egg. Mixing the eggs in thoroughly ensures that the dough emulsifies properly, which prevents the cookies from separating during baking. Once the wet ingredients are fully combined, stop the mixer. The dough should look smooth and glossy at this stage. If you are making dietary substitutions, such as using flax eggs, add them now and allow the mixture to sit for a minute to let the thickeners activate.

4-Fourth Step: Combining Dry Ingredients In a separate medium bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt. Whisking these dry ingredients together distributes the leavening agents evenly, so you do not end up with a bitter bite of baking soda in one cookie. With the mixer on low speed, gradually add the dry flour mixture to the wet butter mixture. Mix only until the flour streaks disappear. Overmixing at this stage develops gluten, which can make the cookies tough instead of tender. The dough will be thick and slightly sticky.

5-Fifth Step: Incorporating the Candy Remove the bowl from the mixer. Using a rubber spatula, fold in the M&M candies by hand. This gentle folding prevents the candy shells from cracking too much before they hit the oven, though some cracking is inevitable and adds to the rustic look. Reserve a handful of candies to press onto the tops of the dough balls before baking for a professional appearance. If you want to mix things up, this is also the time to add other mix-ins like chocolate chips or nuts, adjusting the flour slightly if the dough gets too wet.

6-Final Step: Scooping and Baking Scoop about 1.5 tablespoons of dough and roll them into balls. Place them on the prepared baking sheets, leaving about 2 inches of space between each ball to allow for spreading. If you have time, chill the dough balls in the refrigerator for 30 minutes; this solidifies the fat and results in thicker cookies with a richer flavor. Bake the sheets one at a time in the center of the oven for 10 to 12 minutes. The edges should look slightly set and golden brown, but the centers will still look soft and underbaked. Remove them from the oven and press the reserved M&Ms into the warm tops. Let them cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.