Preheat oven to 175 degrees C. Grease and flour an 8 inch square pan and line with butter paper.
Place a medium heatproof 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.
Whisk in eggs, one after the other and add the vanilla essence before adding the flour. Mix in cocoa, flour, salt and baking powder until no trace of flour is left, using a wooden 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 (I sprinkled some brown sugar on top). Store in an airtight container