Preheat your oven to 175°C. Grease a loaf pan and line it with parchment paper, leaving some overhang for easy removal.
Beat the eggs together with the granulated sugar, vanilla sugar and salt for a few minutes on high speed. It should have a frothy effect.
Add the sunflower oil and milk. Mix until combined. Add the flour and baking powder and mix everything until smooth.
Divide the batter by two (665 gr and 300 gr).
Add 30 gr of cocoa to the 300 gr batter and mix it together until combined.
Pour the vanilla batter of 665 gr into the baking pan. Now add the chocolate batter over it again.
Use a butter knife or skewer to gently swirl the batters together, creating a marbled pattern.
Place it in a preheated oven at 175 °C for about 38 to 45 minutes. or until a skewer inserted into the center of the cake comes out clean.
Once baked, remove the marble cake from the oven and allow it to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
Once cooled, slice the marble cake into thick slices and serve. Enjoy with a cup of tea or coffee for a delightful treat!