Cream the butter and sugars in a stand mixer on medium-low until smooth and creamy, about 1 minute.

Add the egg and vanilla extract to the mixture and mix on low until incorporated.

In a separate bowl, whisk together kosher salt, cornstarch, all-purpose flour, and cake flour. Gradually add to the wet ingredients.

Fold in the chopped chocolate until evenly distributed.

Shape the dough into a 9 to 10-inch log, wrap tightly and refrigerate for at least 2 hours.

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

Slice the dough into 1/4-inch thick rounds and place on the prepared baking sheet.

Bake for 10 to 13 minutes until edges are lightly golden. Watch for the centers to remain soft.

Cool the cookies on the baking sheet for 5 minutes, then transfer to a wire rack to cool for at least 15 minutes before serving.