Method
Grease the tin with some butter and line with non-stick baking parchment. Set the oven to 170 degrees centigrade (160 fan).
Get going by leaving the eggs and sugar to mix together in an electric mixer.
Keep about two squares (10-15g) of the chocolate to one side. Break the rest of the chocolate into pieces and place in a heatproof (eg Pyrex) bowl with the butter.
Stir and melt (do not boil) the contents into a sauce by placing the bowl over a bain marie and then remove from the heat.
Sieve the flour and cocoa powder into a separate bowl. Add the pinch of salt and mix together.
Add the vanilla essence to the sugar/eggs and continue to mix until everything in the bowl is fluffy and creamy (you may need a medium/high speed), and then remove the bowl from the mixer.
Add about a third of the chocolate sauce to the mixing bowl and fold together, then repeat until all the chocolate sauce has been added.
Then add the flour and cocoa powder and fold together until evenly mixed.
Pour the mix into the baking tin and spread evenly. Roughly chop the two remaining squares of chocolate and sprinkle on top.
Place the tin in the centre of the oven for 25 minutes or until cooked through.