Melt the unsalted butter in a microwave-safe bowl for about 30 seconds, then set aside to cool slightly.

Whisk together the flour, baking soda, cornstarch, and salt in a large mixing bowl.

In a stand mixer, combine the melted butter, granulated sugar, and brown sugar. Beat until creamy, then add orange zest and vanilla extract.

Gradually mix the dry ingredients into the wet mixture until a soft dough forms.

Chop the orange dark chocolate bar into chunky pieces and fold into the dough.

Cover the dough with plastic wrap and chill in the refrigerator for at least 2 hours.

Preheat your oven to 350°F (175°C) and line the baking sheets with parchment paper.

Scoop the dough into balls and place them on the prepared baking sheets, leaving space between each ball.

Bake for 11 to 13 minutes until the edges are golden brown.

Cool the cookies on the baking sheet for 3 to 4 minutes before transferring to a wire rack.