Whip the eggs and sugar in a bowl. Pour in the yoghurt, which you've beaten with a pinch of baking soda, add the oil. Stir well with a fork.
Add the vanilla, coarsely chopped hazelnuts and finally, the flour with baking powder in 2 lots.
Stir well again and set aside 1/2 cup from the mixture. Mix in the cocoa and chocolate spread to it to obtain a smooth mixture.
Pour half the white mixture into a rectangular cake form, pour 4-5 spoonfuls of the chocolate one on top in circles. Slide a knife across to get a nice marble-like design.
Next, pour in white mixture again, chocolate on top and make the same marble design.
Bake in a preheated 320°F (160 °C) oven with the fan on for 35-40 min., until your toothpick comes out dry when you poke it.
Take it out and cut it into pieces once cooled.
A fantastic cake for coffee, tea and celebrations!