Preheat the oven to 180°C and grease and flour a round cake pan with a hole in the center.
In a bowl, combine the flour and baking powder. Mix well.
In another bowl or in the mixer's bowl, add the sunflower seed oil, sugar, and orange zest. Beat well until the sugar is dissolved. Add the eggs one by one, beating well after each addition. Beat until the mixture becomes fluffy. Reduce the mixer's speed and pour in the milk and orange juice.
Gradually add spoonfuls of the flour mixture. Pour half of the batter into the cake pan. To the remaining batter, add the cocoa powder and 1/3 cup of milk. Mix with the mixer until well combined. Pour the cocoa mixture into the cake pan. Use a fork to create a marbled pattern by gently swirling the two mixtures together.
Bake in the preheated oven for 50 minutes. Allow the cake to cool, then remove it from the pan. Sprinkle powdered sugar on top and serve