Preheat the oven to 350℉/176℃ and line two baking trays with parchment paper and set aside.
Brown the butter: In a pot over medium-high heat, add the butter and stir until melted. It will foam, sizzle, and crackle (3-5 minutes), continue stirring the entire time to prevent burning. The butter will turn an amber brown color, and the aroma of nutty browned butter along with specks at the bottom of the pan will arrive when it is ready. Transfer into a heat-safe glass to cool for 15 minutes while you prep the rest of the ingredients.
1/2 cup + 2 tbsp unsalted butter
In a bowl, add the oats, flour, baking soda, baking powder, cornstarch, and salt. Stir to combine and set aside.
1 1/4 cups all-purpose flour, 1 1/2 cups rolled oats, 1 tsp cornstarch, 1/2 tsp baking soda, 3/4 tsp baking powder, 1/4 tsp salt
In another large bowl, add the brown butter, brown sugar, and white sugar. Whisk to combine.
1/3 cup dark brown sugar, 1/4 cup white sugar, 1/2 cup + 2 tbsp unsalted butter
Add in egg, yolk, and vanilla. Continue whisking for another minute or so until well combined.
1 large egg, 1 large egg yolk, 2 tsps vanilla
Add in the dry ingredients. With a spatula, gently fold just until a soft cookie dough forms. Do not over-mix!
Add in the M&M's and chocolate chips. Continue mixing until no flour streaks appear.
1/2 cup Valentine's Day M&Ms, 1/2 cup semi-sweet chocolate chips
Scoop 6 cookies per tray, spacing them evenly apart from one another. Add extra M&M's and chocolate chips on top if desired.
Bake for 13-16 minutes or until the cookies are baked on the edges, and still slightly puffy in the middles. Allow to cool, and enjoy!