Watch this video to learn how to make marble cake. Stir margarine and sugar light and porous. Add the eggs, one at the time. Stir well between every egg. Mix in wheat flour with baking soda and skinned milk. The pastry should be well blended. Take one third of the pastry and mix in cocoa, vanilla sugar, sugar and skinned milk. The cocoa pastry should have the same consistency as the light pastry. Put half of the light pastry in the prepared pan and allot it even outward. Then put in the cocoa pastry and in the end the rest of the light pastry. Take a fork or a spoon through all the three layers of pastry, from the one short side of the pan to the other. This way, the cake will have a "marble look". Bake the cake in the oven for about 40 minutes (Temperature 175 Celsius degrees) Let the marble cake cool for a few minutes in the pan. Then carefully remove the cake from the pan and let it cool on a wire rack for a few minutes more before slicing.