In a stand mixer fitted with the paddle attachment, combine the unsalted butter, granulated sugar, powdered sugar, and orange zest. Mix on medium-low speed until smooth and creamy, about 1 minute.

Increase the mixer speed to medium-low and add the egg and vanilla extract. Mix until well incorporated, about 30 seconds.

Gradually add the kosher salt, cornstarch, all-purpose flour, cake flour, and finely chopped chocolate. Mix on low speed until just combined.

Transfer the dough onto a clean surface and shape into a log about 9 to 10 inches long. Wrap tightly in parchment paper and refrigerate for at least 2 hours.

Preheat your oven to 350°F (175°C). Line a baking pan with parchment paper.

Once chilled, remove the dough from the refrigerator and unwrap. Slice the log into 1/4-inch thick cookies and place on the prepared baking sheet.

Bake in the preheated oven for 10 to 13 minutes until edges are lightly golden and cookies appear soft in the center.

Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.