Mix flour, baking powder and salt in a medium size bowl. Set aside to wait.
In a separate medium size bowl, mix 1.25 dl of the sugar, all cocoa powder and the water until the mixture is smooth. Set aside.
Whisk melted butter, rest of the granulated sugar and vanilla sugar until the mixture is blended. Add one egg at a time, mixing well after each addition. After adding the eggs, the mixture should be light and fluffy.
Add the flour mixture and the milk in 3 turns. Always mix until blended.
Add about 4.75 dl of the batter to the cocoa powder mixture and stir until blended.
Prepare a loaf pan (or a bundt cake pan). Pour a thin layer of the light vanilla mixture on the bottom of the pan. Add another thin, uneven layer of the cocoa mixture on top. (The cocoa layer doesn’t have to cover the vanilla layer.) Repeat until the pan is almost filled.
Bake the zebra loaf cake for about 60-75 minutes or until a cake tester comes out clean. (The right baking time may vary a lot depending on your oven). Start checking your cake after 50 minutes. If the cake starts to get too dark brown before it’s cooked inside, cover the cake with aluminun foil. Let the cake cool before serving.