Separate the egg yolks from the whites. Beat the yolks with the sugar. Add the butter and flour with baking powder.
Beat the whites and add them to the yolk mixture. Divide it up into 3 parts. Add the poppy seeds to 1 part, the cocoa to the next and leave the 3rd one yellow.
Pour the cocoa mixture lengthwise down the middle of a rectangular cake form. Add the poppy seed mixture on both sides, without covering the cocoa one, and pour the yellow one over the cocoa mixture.
Bake at 356°F (180 °C) for 35-40 min. Leave it to cool slightly and serve.