Cream together the unsalted butter, granulated sugar, powdered sugar, and orange zest on medium-low speed for about 1 minute until smooth.

Add the egg and vanilla extract to the creamed mixture and mix on low speed until fully combined, about 30 seconds.

In a separate bowl, whisk together the kosher salt, cornstarch, all-purpose flour, and cake flour. Gradually combine this mixture into the wet ingredients until a soft dough forms, approximately 1 minute.

Gently fold in the chopped chocolate evenly throughout the dough without overmixing.

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

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

Slice the chilled dough into ¼-inch rounds and arrange them on the baking sheet with space in between each.

Bake in the preheated oven for 10-13 minutes until the edges are golden.

Transfer to a cooling rack and let cool for approximately 15 minutes before serving.