Preheat the oven to 180 °C. Line a 23 x 32cm slice tin with baking paper.
Combine the dark chocolate (chopped), cocoa powder and coffee powder together in a large heat-proof bowl.
Melt the butter in a large saucepan over a medium-low heat. Cook for 5-10 minutes, stirring frequently. When the butter starts to form a foam, watch as you stir, until the butter turns brown. Remove from the heat immediately at this point so it doesn't burn.
Pour the butter into the chocolate mixture and stir until smooth. Set aside to cool for 10-15 minutes.
Add the sugars, vanilla and eggs to the bowl of a stand mixer. Whip on a medium -high speed for 4-5 minutes, until thick, fluffy and light in color.
Reduce the speed and then pour the chocolate mixture into the sugar mixture. Mix until fully incorporated.
Add the flour and salt into the mixture, and use a spatula to fold through. It's important not to overmix the flour here.
Transfer the batter to the prepared slice tin and spread evenly.
Bake for 25-27 minutes, until just set (the middle shouldn't be too jiggly).
Leave to cool for at least 30 minutes before remove from the pan and slicing.