Grease an 11" square baking tin with softened butter and line with a sheet of parchment paper. Set aside.
Chop the dark chocolate into small pieces.
In a heatproof mixing bowl, combine the chopped chocolate pieces, 25g of the cocoa powder and espresso powder.
Melt the butter in a microwave for 20 seconds each time until it is just simmering hot.
Pour the hot melted butter into the chocolate mixture and let it sit for about 3 minutes. Gently stir the mixture until it is completely smooth. Set aside.
Combine granulated white sugar, dark brown sugar, eggs and vanilla extract in a mixing bowl (either a stand mixer with a paddle attachment or a hand mixer). Beat it at medium speed until fluffy and light, just like a pancake batter, for about 8-10 minutes.
Switch to the low speed, pour the chocolate mixture into the fluffy egg mixture gently and continue to mix them until they are well combined.
Sift the flour and add 50g of the cocoa powder and salt in a bowl.
Use a rubber spatula to gently fold the flour mixture into the fluffy egg mixture.
Preheat the oven at 180°C.
Pour the batter into the prepared tin and smooth the top with an offset spatula. Bake it for 20 minutes or until slightly raised and puffed up on top.
Remove the pan from the oven using oven mitts. Drop it from a height of about 12" onto a heatproof surface. Do this 2-3 times until the brownie slightly deflates. Sprinkle sparingly with some sea salt flakes (don't overdo it - you can adjust to taste over time).
Return the brownie to the oven and continue to bake for another 10 minutes. To know when they are ready, insert a skewer into the centre of the brownies ; it should come out slightly dirty with fudge but without any wetness. Don't over-bake as brownie will continue to cook as it cools.
Use the parchment paper to lift up the large brownie and cool it on a wire rack for 30 minutes. When cool, remove the parchment paper and cut into 16 equally sized brownies.
Serve immediately and enjoy!