In a large mixing bowl (that's heat safe) add the cocoa powder and pour the hot browned butter over the top, whisking to thoroughly combined. Add the chopped chocolate, stirring to combine. Set aside, stirring frequently until smooth and all of the chocolate is melted.
In the bowl of a stand mixer fitted with the whisk attachment, add the eggs, sugar, brown sugar, vanilla and salt and whisk on medium-high for about 5 to 7 minutes or until thick and light in color. Add in the melted chocolate-butter mixture and run the mixer for 30 seconds on low to combine. Take the bowl out of the mixer and use a rubber spatula to fold in the flour until just combined (make sure there aren't any flour pockets). Pour the batter into the prepared baking dish.
Bake on the middle of the preheated oven for 25 to 27 minutes. When done, the tops will be shiny and if you test with a toothpick moist crumbs will cling to it. Note: the crinkly top won't show up until you pull them out of the pan and cut them. Let them cool in the pan for several hours and if you want super, uber fudgy brownies shove them in the fridge for several hours when cooling. Sprinkle with the sea salt flakes, if using.
When it’s time to cut the brownies, use a very sharp knife and use a wet paper towel to clean off the knife in between cuts. Enjoy!