In a medium bowl, whisk together flour, baking soda, salt, and cornstarch.
In a large bowl, cream cold butter, brown sugar, and sugar until light and fluffy.
Add eggs and vanilla extract, mixing until combined.
Gradually add dry ingredients and mix until just combined.
Fold in chocolate chunks.
Cover and chill dough for at least 1 hour, preferably overnight.
Preheat oven to 400°F and line baking sheets with parchment paper.
Scoop large dough balls, about ¼ cup each, onto prepared baking sheets.
Top with extra chocolate chunks if desired.
Bake for 11–14 minutes, until edges are golden but centers still look slightly underdone.
Sprinkle with flaky sea salt and let cool on the baking sheet for 10 minutes before transferring.