Preheat the oven to 180ºC and line your baking pan with parchment paper.
In a large bowl, add the chopped chocolate and 1/2 of the cocoa powder (2tbsp).
Heat the butter in a saucepan, for about 2-3 minutes, until it begins to boil
Remove from the heat, and pour over the chocolate and cocoa powder
Whisk until smooth and the chocolate has completely melted. Keep this mixture aside
In a large, separate bowl, add in the castor sugar, brown sugar and eggs.
Using an electric hand beater, beat this mixture for atleast 7-8 minutes.
Add in the vanilla essence and beat again until well combined.
Now, add in the chocolate and butter mixture, and beat for another 1 minute
Sift in the flour, salt and the rest of the cocoa powder.
Switch to a spatula and fold in these dry ingredients.
Bake for 20 minutes.
After 20 minutes, remove it from the oven, and tap the baking tin on your kitchen surface 3-4 times. This gives you the classic crackly top.
Bake again for 20 minutes.
Cool completely before cutting into pieces.
