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, baking powder, and vanilla, and mix well.
2. In another bowl or the mixer bowl, add the sunflower oil, sugar, and orange zest. Beat well until the sugar dissolves, then add the eggs one by one, beating well after each addition. Beat until the mixture is fluffy. Lower the mixer speed and add the milk and orange juice.
3. Add the flour mixture tablespoon by tablespoon. Pour half of the batter into the pan. To the remaining batter, add the cocoa and the 1/3 cup of milk. Mix with the mixer until incorporated. Pour the chocolate batter into the pan and use a fork with circular motions to create 'swirls' in the cake. Bake in the preheated oven for 50 minutes. Let it cool, unmold, sprinkle with icing sugar, and serve.
