Preheat your oven to 175°C (350°F), and line a 9 x 5-inch loaf pan with parchment paper with enough overhang that you can easily pull the brownies out of the pan after they're baking.
Melt the butter, chocolate and cocoa powder in a double boiler or in a heat proof bowl over a pot of simmering water, making sure the bottom of the bowl doesn't touch the water. Remove from heat once everything has melted, about 5 minutes.
In another bowl, mix the egg, brown and white sugar, salt and vanilla together.
Pour in the slightly cooled chocolate mixture and stir to combine.
Finally, add the flour and stir until just combined. Don't overmix!
Spread the batter into your prepared loaf pan, smoothing it out into all the corners.
Bake for 20-22 minutes. You want them slightly underbaked because they will go back into the oven again later. The surface of the brownies should be shiny with a crackled top.
Lift the brownies out of the loaf pan out using the parchment paper and cool completely. If you want to speed up the process, you can pop the brownies in the fridge.
Brownies can be made a day in advance and stored in an airtight container after completely cooled.