Brown the Butter: In a light-colored pan over medium-high heat, melt the butter and continue cooking for about 5 minutes until golden brown specks form at the bottom. Remove from heat and let it cool for 35-45 minutes.

Mix the Wet Ingredients: In a mixing bowl, beat the cooled brown butter, granulated sugar, and brown sugar until light and fluffy. Add the egg and mix until fully incorporated, then mix in the egg yolk and vanilla extract.

Combine the Dry Ingredients: In a separate bowl, whisk together the flour, salt, and baking soda.

Form the Dough: Gradually mix the dry ingredients into the wet ingredients until just combined. Avoid overmixing. Fold in half of the M&Ms, reserving some for topping.

Portion the Dough: Scoop out 80-gram portions of dough (about ¼ cup each) and roll into balls. Press additional M&Ms on top.

Chill the Dough: Refrigerate the dough balls for at least 1 hour, or up to 72 hours for enhanced flavor.

Bake: Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper. Place 4 cookie dough balls per sheet, spacing them apart. Bake for 13-14 minutes, until the edges are golden brown but the centers remain soft.

Cool & Enjoy: Let the cookies cool completely on the baking sheet before serving.