Line a greased baking tray with butter paper.
Sieve the flour and baking powder together.
Melt the butter and mix in the cocoa.
Let it cool and mix in the essence.
Break the eggs into a bowl, whisk and sugar well.
Mix in the cocoa mixture which should be cool by now.
Gently, fold in the flour and walnuts.
Pour into the prepared tray and bake in an oven till cooked and golden brown.
Cook over a temperature of 240C or 450F for 10 or 15 minutes.
When cold, remove from tray and cut into squares.