For the marble cake, in a bowl over hot steam, beat yolks with sugar, water, vanilla pulp, lemon zest and a pinch of salt until foamy. Lift out of the water bath and beat until cold.
Slowly pour in oil.
Beat egg whites until stiff and fold in alternately with flour mixed with baking powder.
Butter a loaf pan, sprinkle with flour and color about 1/3 of the mixture with sifted cocoa powder. Pour in about 1/3 of the light mixture first, followed by about 4 tablespoons of cocoa mixture, then the remaining light mixture and again the remaining cocoa mixture. Using a wooden spoon, very gently run it through the mixture with a gentle twisting motion to achieve the marbling typical of a marble cake.
Bake in the preheated oven at 170 °C top and bottom heat for approx. 40 minutes. Turn out the marble cake and sprinkle with powdered sugar.