Mix eggs, lemon peel, vanillin sugar and condensed milk. Stir in oil. Mix flour and baking powder and stir into the mixture.
Cut the dough in half. Stir in half of the cocoa and milk. First fill the light, then the dark dough into a greased and flour-spread springform pan (approx. 26 cm Ø) with a tube bottom insert and swirl gently with a fork.
Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 40-45 minutes.
Take the cake out of the oven, let it cool down for about 20 minutes and turn it onto a cake rack. Let the cake cool down. Dust with icing sugar before serving.