Grease a 9"x9" baking pan butter, then line with baking paper, leaving overhang on all sides. Set aside.
Combine the chopped chocolate, 2 Tbsp. of cocoa, and espresso powder in a medium sized bowl and set aside.
Melt the butter over a medium heat and cook until the butter is melted and hot. Pour the hot butter straight over the chocolate mixture and let sit for 2 minutes. Mix until the chocolate is completely smooth and melted, then set aside.
Combine the sugar, brown sugar, vanilla extract and eggs in a large bowl. Beat with an electric hand mixer on high speed until light and fluffy, about 10 minutes. It will triple in size.
Beat the chocolate mixture through the egg mixture.
Fold through the flour, remaining cocoa and salt until just combined.
Preheat the oven to 180°C (350°F) and pour the brownie batter into the prepared baking pan. Bake until lightly puffed on top, (around 20 minutes.)
Remove the brownies from the oven, then drop the pan on a flat surface 1-2 times until the brownies deflate. Sprinkle with flaky sea salt (optional).
Return to the oven and bake until a wooden skewer inserted into the center of the brownies comes out fudgy but the edges look cooked through, around another 20 minutes.
Allow the brownies to cool completely in the pan and then lift them out and slice!