In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar. You can do this using a stand mixer with a paddle attachment or an electric hand mixer (the mixture should be lighter in color and smooth in texture after this step).
Add in the room temperature egg and the vanilla extract and mix well to combine.
In a separate mixing bowl, whisk together the flour, cornstarch, salt, and baking soda.
Add half of the dry ingredients to the butter mixture and mix on low speed until combined.
Next, add in the remaining dry ingredients and mix until just incorporated.
Roughly chop the chocolate bar and fold the chopped chocolate into the cookie dough until it's evenly distributed.
Using a large cookie dough scoop, scoop out 15 cookie dough balls and place them on a parchment paper-lined baking sheet or a large plate.
Wrap this baking sheet or plate in plastic wrap and place it in the fridge for at least 6 hours or overnight.
Once you're ready to bake the cookies, preheat the oven to 375℉ and line a baking sheet with parchment paper (if you're baking all of the cookies at once, you will need two or three pans depending on the size).
After the cookie dough balls have finished chilling, roughly chop more chocolate and top each cookie dough ball with those chocolate pieces.
Place 6 cookies on your baking sheet a few inches apart and bake at 375℉ for 10-13 minutes or until the edge of each cookie is just slightly golden brown. (The baking time will vary depending on your oven as well as how long you chilled the dough!)
After you pull the cookies out of the oven, sprinkle each one with coarse or flaky sea salt and enjoy!
