Simmer the butter, sugar, water and cocoa for 3-4 minutes. Set aside 1/4 of the mixture. To the remainder add two beaten egg yolks and the flour sifted with baking powder.
Finally, add the two beaten to snow egg whites and mix gently. Spread the mixture into a tray, pre-greased and lightly floured, or laid out with baking paper.
Bake about 20 minutes and check to see if it's ready with a toothpick. 5-6 minutes after removing it from the oven, brush the sides with the cocoa mixture.