1. Preheat the oven to 180°C (350°F) and grease and flour a round cake pan with a hole in the middle. In one bowl, combine the flour and baking powder and mix them together.
2. In another bowl or in the mixer bowl, place the sunflower oil, sugar, and orange zest. Beat well until the sugar dissolves, then add the eggs one by one, beating well before each addition. Beat until the mixture is fluffy. Lower the mixer speed and add the milk and orange juice. Add the flour mixture spoon by spoon. Pour half of the batter into the prepared pan.
3. To the remaining batter, add the cocoa powder and the 1/3 cup of milk. Mix with the mixer until incorporated. Pour the cocoa batter over the light batter in the pan and use a fork with circular motions to create a marbled effect ('waters') in the cake. Bake in the preheated oven for 50 minutes. Let it cool, turn it out onto a plate, dust with icing sugar, and serve.
