First of all grease the 12 x 8 inch pan. In a bowl put butter, sugar and vanilla essence, beat with electric beater until becomes cream. Take another bowl and strain flour, coco powder and baking powder. Now add eggs one by one with 1 tbsp of dry mixture and beat well. Now add remaining dry mixture and walnuts, mix well. Transfer this mixture in greased cake pan and bake in pre heated oven on 180ºC, 350ºF and Gas Mark 4 for 35 to 40 minutes. When it’s done, let It cool down. Cut into squares and serve.