In a saucepan, melt the butter over low heat. Bring to a simmer and simmer until the butter starts to brown and gives off a nutty aroma. Pour the butter into the bowl of a stand mixer, discarding the darkest bits that are at the bottom of the pan, and allow to cool slightly, about 5 minutes.
In the meantime, in a large bowl, whisk together the flour, cornstarch, salt, baking powder and baking soda for 30 seconds to evenly distribute the leavening. Stir in the grated unsweetened chocolate.
Add the brown and granulated sugar to the browned butter and mix until smooth and slightly fluffy. Add the eggs one at a time, mixing until just combined. Scrape down the bowl, add the vanilla and briefly mix. Add the flour mixture and mix on low speed until the dough just comes together. Add the chocolate morsels and chopped chocolate and mix until just combined. Cover with plastic wrap and refrigerate at least 20 minutes if you are baking the same day (or allow to stay covered in the fridge for 12 to 24 hours if baking later).
Preheat the oven to 375 degrees F. Line 2 baking sheets with parchment paper.
Scoop the dough with a medium cookie scoop (1.5 ounces) and place 3 inches apart on the prepared baking sheets; press the cookies very slightly to flatten. Bake, opening the oven and gently slamming the baking sheets halfway through the baking process, until the edges are slightly golden brown, 10 to 12 minutes.
This recipe has been updated and may differ from what was originally published or broadcast.