Preheat the oven to 180°C.
Line a rectangular baking tray with baking paper.
Melt the butter in a small sauce pan over low heat. Do not let it brown. Set aside to cool.
In a large bowl, sift the flour, cocoa and sugar. Add nuts (if using). Mix well.
Break the chocolate into small pieces (you can buy chocolate drops to make this part quicker).
Stir in the lightly beaten eggs, vanilla and butter. It is important that the butter is not too hot as it will l cook the eggs. Stir to combine.
Pour the mixture into the tin; smooth out the surface. Bake for 45 minutes.
Allow to cool in the tin before cutting it into squares. Dust with a little icing sugar, if you wish.