Combine the chopped chocolate, 1/4 cup of the cocoa powder in a heatproof liquid measuring cup or medium bowl and set aside
Add the butter to a small saucepan over medium heat and cook until the butter just comes to a vigorous simmer about 5 min swirling the pan occasionally. Immediately pour the hot butter over the chocolate mixture and let sit for 2 min. Whisk until the chocolate is completely smooth and melted and set aside.
Combine the granulated sugar, brown sugar, vanilla extract, salt and eggs in a large bowl. Beat with an electric hand mixer on high speed until light and fluffy, about 10 minutes. It would be similar to the texture of a very thick pancake batter.
With the mixer on, put in the slightly cooled chocolate and butter mixture and blend until smooth.
Grease a 9 x 13 inch or 23 x 33 cm dark metal pan with softened butter then line with parchment paper leaving overhang on all sides. Grease the parchment with softened butter.
Position a rack in the middle of the oven and preheat to 180°C or 350 Fahrenheit
Sift in the flour and remaining coco powder￼ and use a rubber spatula to gently fold￼ until combined￼
Pour the batter into the prepared baking pan and smooth the top with a spatula. Bake until lightly puffed on top about 20 minutes￼
Remove the baking pan from the oven using oven mitts or kitchen towels￼, then lightly drop the pan on a flat surface for 1 to 2 times until the brownies deflate slightly. Sprinkle with flaky sea salt
Return the pan to the oven. Bake until a wooden skewer inserted into the center of the brownies come out fudgy but the edges look cooked for about 20 minutes more.