1-First Step: Mise en place Measure all ingredients accurately. Soften butter to room temperature, about 65°F, for easy creaming. Chop extra M&M’s if desired for more chocolate pools. Set up two bowls: dry and wet. This organization prevents rushed mistakes and ensures even mixing.

2-Second Step: Mix dry ingredients Whisk flour, baking soda, and salt in a medium bowl. Sift if lumpy for smoothness. Baking soda reacts with brown sugar acids for lift. Set aside. This dry mix distributes leavening evenly, avoiding bitter pockets.

3-Third Step: Cream butter and sugars Beat butter, white sugar, and brown sugar 2-3 minutes until pale and fluffy. Scrape bowl sides. Creaming traps air for tenderness. Brown sugar’s moisture promises chew. Use paddle if mixer, or sturdy spoon.

4-Fourth Step: Add wet ingredients Mix in egg, egg yolk, and vanilla until smooth, about 1 minute. Egg yolk’s fat enhances richness. Avoid overmixing to prevent toughness. Batter looks glossy now.

5-Fifth Step: Combine wet and dry Add dry mix to wet in three additions, stirring gently until just combined. Dough appears shaggy. Stop mixing to keep gluten low for softness.

6-Sixth Step: Fold in M&M’s Gently fold 1 1/2 cups M&M’s with spatula. Reserve a handful for tops. Overfolding melts them prematurely. Colors pop through dough.

7-Seventh Step: Chill dough Cover and refrigerate 1-2 hours. Chilling firms butter, prevents spreading. Flavors meld too. Quick chill in freezer 30 minutes if rushed.

8-Eighth Step: Scoop and shape Scoop 1.5-inch balls (about 2 tbsp each) onto sheets, 2-3 inches apart. Press tops gently with reserved M&M’s. Larger balls yield chewy centers.

9-Ninth Step: Bake Bake 10-12 minutes until edges golden, centers soft and puffy. Rotate sheets midway. Underbake slightly for chew. Cool on pans 10 minutes, then racks.

10-Final Step: Cool and serve Cookies firm as they cool. Enjoy warm or store. Pair with milk for classic combo. Troubleshoot: Flat? Chill longer. Dry? More butter next time.