Cut the chocolate in chunks and melt the butter to allow it to cool.
In a medium-sized bowl, whisk together the flour, baking soda, cornstarch, and salt. Set aside.
In a large bowl, mix together the cooled melted butter with both sugars.
Then, add eggs one at a time and then add vanilla extract. Beat until just combined.
With mixer on low incorporate the flour mixture.
Fold in the chocolate chunks/chips. Cover and refrigerate the dough for about a couple of hours or if you are in a hurry freeze for 15-20 minutes.
Meanwhile preheat the oven to 325F (165C). Line two baking sheets with parchment paper.
Using an ice cream scoop, take equal portions of dough and place on the prepared baking sheets, 1 inch (2.5cm) part.
Bake for 11-13 minutes until slightly golden on the edges. The cookies will look very soft and underbaked. Let the cookies cool on the baking sheets until the cookies are firm enough to remove, probably another 5-10 minutes. While cookies are still warm add some extra chocolate chunks/chips on top if desired, to get a beautiful look. Cool completely on a wire rack.
These cookies can be kept for several days at room temperature and can freeze up to several months.