• Brown & Cool The Butter. Melt the butter in a light-colored saucepan over medium heat, whisking constantly until the butter begins to foam. (Watch carefully when the butter begins to foam, as it will turn golden and brown quickly.) When the butter under the foam is golden (like honey) and smells sweet with a nutty aroma, remove the pan from the heat and transfer to the bowl of your stand mixer or a large mixing bowl to cool for 10-15 minutes, or until it's no longer hot to the touch. (Make sure to get all those lovely brown bits!)
• Combine Brown Butter & Sugar. When the browned butter has cooled, add the sugar and brown sugar. Mix with the paddle attachment on medium speed 2-3 minutes or until well combined and the mixture looks like wet sand.

Add Eggs + Vanilla Add eggs + egg yolk one at a time, blending well after each addition, then add the vanilla.

• Add Dry Ingredients. Add flour, cornstarch, baking soda, baking powder, and salt. Mix for a few seconds (it won't be fully combined)
• Add Mix-Ins. Scrape down the sides of the mixing bowl, then add the Valentine's M&M candies and chocolate chips. Mix until just combined.
• Scoop & Chill The Dough. Use a medium cookie scoop (2 Tablespoon size) to scoop cookie dough into 2-Tablespoon balls. Place in an airtight container, cover, and chill the cookie dough balls for at least 2 hours, up to 1 day in advance.
• Preheat The Oven & Prep Your Pans. When you're ready to bake, remove the dough from the refrigerator while the oven fully preheats to 350 degrees F. Line two baking sheets with parchment (so you can alternate sheets between batches.)
• Bake The Cookies. Roll 6 cookie dough balls in between your palms to smooth them out and place onto a prepared baking sheet. Bake at 350 degrees for 8-10 minutes, or until the edges are a golden brown color and the centers look just underdone. (Bake closer to 10 minutes for crispier cookies)
• Finish The Cookies. While the cookies are hot, top them with additional M&Ms and chocolate chips, then sprinkle the top of the cookie with flaky sea salt, if desired. (For perfectly round cookies, you can also use a large round cookie cutter, to gently shape the cookies into perfect circles.) Cool cookies on the baking sheet for 2 minutes, then transfer cookies to a wire rack to finish cooling. Repeat with remaining cookie dough.
• Serve & Store. Enjoy right away once the cookies are cooled, or store in an airtight container 2-3 days. (Or, freeze up to 2 months)