Cream the unsalted butter, granulated sugar, powdered sugar, and orange zest in a stand mixer on medium-low speed for about 1 minute until smooth and creamy.

Add the egg and vanilla extract, mixing until just combined (about 30 seconds on medium speed).

Gradually add kosher salt, cornstarch, all-purpose flour, cake flour, and chopped chocolate, mixing on low for 1-2 minutes until the dough forms.

Shape the dough into a log about 9-10 inches long, wrap in plastic wrap, and refrigerate for at least 2 hours.

Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.

Slice the chilled dough into 1/4-inch thick cookies and arrange them on the prepared baking sheet.

Bake cookies for 10-13 minutes or until edges are lightly golden, then cool for at least 15 minutes.