Brown the butter over medium heat in a saucepan until amber and fragrant, about 5 minutes. Transfer to a bowl.

Pour the browned butter into a mixing bowl, scraping in browned bits. Allow to cool slightly.

Add the egg, brown sugar, granulated sugar, and vanilla. Whisk until combined.

In a separate bowl, whisk together flour, cornstarch, baking soda, and salt. Gradually mix into wet ingredients.

Fold in orange M&Ms, black M&Ms, and chocolate chips.

Using a cookie scoop, form dough into 16 mounds, roll into balls, and flatten halfway.

Place on a tray, cover, and refrigerate for at least 4 hours.

Preheat oven to 350F. Line baking sheets with parchment or silicone mats.

Arrange cookie dough on sheets, spaced at least 2 inches apart. Bake 10-12 minutes until edges are set.

Allow cookies to cool on baking sheet for 10 minutes before transferring to a rack.