Adjust oven rack to the middle position. Heat to 375°F (191ºC).
In a medium bowl, whisk together the flour and baking soda, set aside.
Cut the butter into 14 even-sized pieces. In a medium skillet, melt 10 tablespoons of butter over medium-high heat, swirling to melt evenly. Stir and cook, scraping the bottom of the pan with a spatula until butter is golden brown and nutty in aroma, 1 to 2 minutes. Turn off the heat, and remove pan from stove. Stir in the remaining 4 tablespoons of butter until completely melted.
In a stand mixer fitted with the whisk attachment, add the melted butter to the mixing bowl. Add the granulated sugar, brown sugar, salt, and vanilla. Whisk on medium speed (setting 5) until combined, 30 seconds.
Add the whole egg and yolk to the mixing bowl. Whisk on medium-high speed (setting 8) until smooth with no sugar lumps remaining, about 30 seconds. Let the mixture stand for 3 minutes.
Repeat the whisking and resting process three more times: 30-seconds mixing at medium-high speed and 3 minutes of rest. After four total times, the mixture should be thick, smooth, and shiny.
Using a rubber spatula or wooden spoon, stir in the flour mixture until just combined, about 1 minute. Stir in ¾ cup M&M candies and chocolate chips. Give the dough a final stir to ensure no flour pockets remain.
Transfer the mixing bowl to the refrigerator for 30 minutes or freezer for 15 minutes, or until the dough firms up, around 65 to 70ºF (18 to 21ºC). The dough should not feel overly sticky.
Divide the dough into 16 portions, each about 3 tablespoons (about 60 grams, 2 ounces), and roll into a ball. If the dough starts to warm and get sticky, place it on a sheet tray and refrigerate for 15 minutes.
For a rustic appearance, tear the dough balls in half. Press the two halves together with the lumpy torn sides facing upward to expose the chips. Shape the sides of the dough back into a circle. Use the remaining ¼ cup of M&M candies and press some on top, so they pop out of the surface.
Arrange dough balls at least 2-inches apart on lined baking sheets, 8 portions per tray. Smaller baking sheets can be used but will require baking in 3 batches. The dough should be room temperature or just below for even cooking, around 65 to 70ºF (18 to 21ºC).
Bake one tray at a time in the center position for 5 minutes, then rotate the baking sheet. Continue to bake until the edges are golden brown and the centers look slightly underdone, about 5 to 7 minutes. The color will continue to darken during cooling. Place the baking sheet on a wire rack and let the cookies cool until they reach room temperature, 20 to 30 minutes.
Make sure the oven returns to 375°F (191ºC) before baking the remaining tray.
Notes
Make ahead: Cookie dough can be shaped into balls and refrigerated in a resealable plastic bag for up to 1 week or frozen for up to 2 months before baking. Break the dough apart and combine right before baking for a rustic surface. Add more M&M candies on top.
Storing: Baked cookies can be stored in an airtight container at room temperature for 5 days or 3 months in the freezer.