Preheat your oven to 160 degrees celcius / 320 faranheit
Place butter into a saucepan over a medium heat and bring to a gentle boil.
Turn heat OFF and then add dark chocolate stirring intermittently until the chocolate has melted. Remove from heat and allow to cool slightly
Place 3 eggs and sugar into a bowl and beat with an electric mixer until they have almost doubled in size and are super light and fluffy. At least 7 minutes
Sift the flour and cocoa into a bowl
Pour the chocolate mixture into the egg mixture and gently fold the two together using big round figure 8 movements. Stop mixing as soon as you no longer see vanilla streaks through the chocolate batter
Re-sift the flour & cocoa evenly over the top of the chocolate mixture and fold together, again using the figure 8 action
**OPTIONAL - add 1/2 cup white chocolate chunks and 1/2 cup milk chocolate chunks, or nuts as desired**
Pour into a brownie tin that's been sprayed with cooking spray and lined on the base with baking paper
Bake for 20-25 minutes. The top should start to look a little flaky and the middle should spring back lightly when touched
Remove from oven and make ganache while the brownie cools.