Whisk in eggs, one after the other and keep this aside
Grease and flour an square pan and line it with butter paper
Preheat oven to 175 degrees C
Place bowl over a pan of simmering water
Place butter in the bowl and let it melt
Remove the bowl from the simmering water and stir in the sugar,It will appear grainy at this point
Now add whisked eggs and vanilla essence before adding the flour
Mix in cocoa, flour, salt and baking powder until no trace of flour is left, using spoon
Spread batter into prepared pan
Bake in preheated oven for 30 minutes
Do not overcook
When you find that the edges have gone slightly dry and the top is cracked, you know its cooked
Cool on a wire rack
Cut into squares at room temperature and top with walnut
Store in an airtight container