To brown the butter, melt the unsalted butter in a shallow, light-colored pan over medium-low heat. Cook, stirring frequently, for 5-7 minutes. At first the butter will foam up and get bubbly, but eventually, you will start to see small brown bits on the bottom of the pan. It’s ready when the butter is golden-brown and smells nutty and caramelized. Immediately remove the butter from the heat and pour it into a large heat-proof bowl to cool for 5-10 minutes.

Meanwhile, whisk together the all purpose flour, baking soda, and Kosher salt in a small bowl. Set aside.

Next, add the dark brown sugar and white sugar to the brown butter. Using an electric hand mixer, beat the butter and sugars together until it’s well-combined, 1-2 minutes. Next add the eggs one at a time until incorporated. Mix in the vanilla extract.

Add half of the flour mixture to the sugar-butter mixture until mixed. Then, add the remaining flour mixture, beating just until the dough forms. Use a rubber spatula to ensure there are no dry spots. The dough will be a bit loose, but it will thicken once it's chilled. Cover the bowl and chill the dough in the refrigerator for at least 1 hour or overnight.

Once the dough is chilled and you're ready to bake, preheat the oven to 350°F. Line two sheet pans with parchment paper. Then, remove the dough from the fridge and gently fold in the unsalted peanuts, semi sweet chocolate chips, and M&Ms until they’re well distributed.

Using a #40 cookie scoop (2 Tablespoons), scoop the cookie dough and roll into balls with the palms of your hands. Place the cookie dough balls 3 inches apart on the parchment-lined sheet pans (they will spread quite a bit).

Bake for 8-9 minutes until the edges are set and the centers are still a bit underdone. Remove the cookies from the oven and immediately press 4-5 M&Ms on top of the cookies if desired (this is just for aesthetic purposes).

Let the cookies cool for 10 minutes on the pan, and then transfer to a wire rack to cool completely. Enjoy!