In a stand mixer, combine room temperature unsalted butter, granulated sugar, powdered sugar, and orange zest. Mix on medium-low speed for about 1 minute until creamy and light. Add the egg and vanilla extract, mixing until smooth.

Add kosher salt, cornstarch, all-purpose flour, and cake flour to the butter mixture in the stand mixer. Mix on low speed until just combined, then fold in the chocolate evenly.

Transfer the dough onto a clean surface and shape it into a 9-10 inch log. Wrap tightly in plastic wrap and refrigerate for at least 2 hours.

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

Remove the dough log from the refrigerator, slice into 1/4 inch thick cookies, and arrange them on the baking sheet. Bake for 10-13 minutes until edges are lightly golden.

Let the cookies cool on the baking sheet for about 15 minutes before transferring to a wire rack.