Chocolate Cake using the ingredients and equipment you provided:

Preheat the Oven:

Preheat your oven to 180°C (350°F). If using parchment paper, line the cake mold with it for easy removal later.
Prepare the Cocoa Mixture:

In a small bowl, combine the cocoa powder and 3 tablespoons of milk. Stir until you achieve a smooth paste. Set this aside.
Mix the Wet Ingredients:

In a large mixing bowl, whisk the eggs with the salt, vanilla sugar (or vanillin), and sugar until the mixture becomes light and fluffy.
Gradually add the sunflower oil and milk, continuing to whisk until everything is well incorporated.
Combine the Dry Ingredients:

In a separate bowl, sift the flour, baking powder, and bitter cocoa together.
Combine Wet and Dry Ingredients:

Gradually add the dry ingredients to the wet mixture, stirring gently with a spatula or using a mixer on low speed. Mix until smooth and just combined.
Add the cocoa paste and mix it into the batter to create a marbled or swirled effect.
Pour into the Mold:

Pour the batter into your prepared plum cake mold or loaf pan. Use a spatula to smooth the top of the batter.
Bake the Cake:

Bake in the preheated oven for about 30-40 minutes or until a toothpick inserted into the center comes out clean.
If needed, cover the top with foil halfway through to prevent over-browning.
Cool:

Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.