Preheat the oven to 350°F (177°C). Grease an 9 x 13–inch (23 x 33–cm) baking dish and line with parchment paper, letting the excess fall over the sides. (see note below regarding pan size)

In a medium, heat-safe bowl add the chocolate, butter and cocoa powder and set over a medium saucepan of simmering water. Do not let the bowl touch the water or let the water come to a boil. Stir frequently until melted and smooth and set aside.

In an electric stand mixer fitted with the whisk attachment, add the eggs, sugar, brown sugar, espresso, 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 (no need to let it cool) and run the mixer for 30 seconds on low to combine. Take the bowl out of the mixer and fold in the flour until just combined (make sure there aren't any flour pockets). Pour the batter into the prepared baking dish. Sprinkle with the sea salt flakes, if using.

Bake on the middle of the oven for 25 to 27 minutes. When done, the tops will be shiny and if you test with a toothpick some 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 before serving.

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!