Pre heat the oven to 160°c. Place the butter and sugar and cocoa in a bowl on a double boiler.( place water in a wide pan. Let it boil. Keep butter n sugar in a bowl on top of the pan. The bottom of the bowl should not touch the water) let the butter melt and mix with sugar and cocoa. Stir till well mixed. To this add eggs one by one. But do not stop mixing as that might just cook the eggs. When the eggs have mixed in well add vanilla essence. Sift the flour over the mixture and whisk well. Pour the batter into a lined pan and bake it for 30 to 35 minutes or until the centre is set. Cool it in the tin and cut into poeces and serve it with icecream or mini meringues.