Place the butter in a microwave-safe bowl and heat for 30–45 seconds until mostly melted with a few soft pieces remaining.

In a large bowl, whisk together flour, baking soda, cornstarch, and salt.

In a stand mixer or large bowl, beat butter and both sugars on medium speed for 1 minute. Add orange zest and vanilla and mix for another minute.

Add eggs one at a time, mixing well between each addition.

Reduce mixer speed to low and gradually add the dry ingredients, mixing just until combined.

Fold in chopped chocolate with a spatula. Cover and refrigerate dough for at least 2 hours or overnight.

Preheat oven to 350°F and line baking sheets with parchment paper.

Scoop dough into 1.5-tablespoon portions and place 2 inches apart on baking sheets. Sprinkle with flaky sea salt if desired.

Bake for 11–13 minutes, rotating the pan halfway through, until edges are golden.

Cool cookies on the baking sheet for 3–4 minutes, then transfer to a wire rack to cool completely.