Start by melting the unsalted butter in a saucepan over medium heat. Stir occasionally until it turns golden brown and smells nutty.
In a large mixing bowl, combine the browned butter, light brown sugar, and granulated sugar. Whisk until smooth and creamy.
Add the vanilla extract, eggs, and egg yolks to the butter-sugar mixture. Whisk until well combined and glossy.
In another bowl, whisk together the all-purpose flour and salt. Gradually add to the wet ingredients, folding gently with a spatula until just combined.
Fold in the M&M candies and semi-sweet chocolate chips evenly throughout the batter.
Preheat oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving some overhang. Pour batter into pan and spread evenly.
Bake for 25-30 minutes or until edges are golden brown and a toothpick inserted in center comes out with a few moist crumbs.
Remove from oven and cool in pan for about 10 minutes. Optionally sprinkle with flaky sea salt before transferring to a wire rack to cool completely.