Brown the butter over medium heat in a saucepan until amber and fragrant, about 5 minutes, stirring occasionally. Remove from heat and transfer to a mixing bowl.

Add the egg, brown sugar, granulated sugar, and vanilla to the browned butter. Whisk until well combined.

In a separate bowl, whisk together flour, cornstarch, baking soda, and salt. Gradually add to wet ingredients, mixing until just combined.

Stir in the orange M&Ms, black M&Ms, and chocolate chips until evenly distributed.

Using a 2-tablespoon scoop, form dough into balls, place on a tray, and flatten slightly.

Refrigerate the dough for at least 4 hours or overnight to ensure proper spreading and flavor development.

Preheat oven to 350°F. Arrange dough balls on a baking sheet lined with parchment paper, spacing them 2 inches apart.

Bake for 11 to 13 minutes, until edges are golden but centers remain soft. Cool on baking sheet for 10 minutes before transferring to a wire rack.