Line and grease a 13x11 inches rectangular pan.
Chop the chocolate on a cutting board.
In a heatproof bowl, combine the butter and chopped chocolate and set it in a sauce pan of barely simmering water (double boiling method).
Stir occasionally until the butter is melted, add in the sugar and continue with the process until the sugar is almost dissolved and the mixture is smooth and a bit shiny.
Remove from heat, add vanilla essence and set aside to cool.
Preheat oven to 170 C.
Sift together flour, cocoa and baking powder.
Once the butter chocolate mixture has cooled, whisk the eggs in a bowl with an electric beater until creamy.
Add a few tablespoons of this into the chocolate mixture (to avoid scrambling) and fold carefully.
Transfer the entire chocolate mixture into the bowl of beaten eggs and start mixing with a hand whisk.
Fold in the sifted ingredients.
Finally, add in the chopped nuts.
Pour the mixture into the prepared baking tray.
Bake for 30 minutes. Read notes*!
Take out the pan from the oven and place on a wire rack to cool completely.
Cut out squares or rectangles of your choice.