Cream the butter, granulated sugar, and powdered sugar until smooth. Add the egg and vanilla extract, mixing until incorporated.

Whisk together the all-purpose flour, cake flour, kosher salt, and cornstarch. Gradually add to the creamed mixture, stirring until a soft dough forms. Fold in chocolate and orange zest.

Shape the dough into a log, wrap in plastic, and refrigerate for at least 2 hours.

Preheat oven to 350°F (175°C). Slice dough into ¼ inch thick cookies and place on a parchment-lined baking sheet. Bake for 10-13 minutes until edges are lightly golden.

Allow cookies to cool on the baking sheet for about 15 minutes before transferring to a wire rack. Store in an airtight container at room temperature.