Whenever I make a cake, I always sift the flour with the baking powder first and prepare the baking form by smearing it with a thin layer of olive oil and sprinkling with flour. Remove any excess flour. Preheat the oven to 356°F (180 °C).
I use regular olive oil that's suitable for baking and doesn't have an olive aroma. Be sure to use one that can withstand heat treatment.
Beat the eggs to a very fluffy cream with the pinch of salt and the sugar, which is added in lots to dissolve easier and for the mixture to fluff up better. Then add the yoghurt, to which you've added the baking soda ahead of time (needs to have bubbled) and stir.
Next add the olive oil and flour with baking powder, also in lots, and stir with a wooden spoon or mixer on low speed. Even if the mixture deflates, it will puff up during baking.
Divide the mixture into 2. Add the cocoa to 1 part and stir until homogeneous. Pour the white part into the baking form and the brown one on top of it.
Bake at 356°F (180 °C) or at 329°F (165 °C) with the fan on for 45 min., or simply until ready - check with a toothpick.
You can always experiment and make the cake with zebra stripes or however you like. Optionally, you can sprinkle with powdered sugar, bake with a syrup according to taste, or with melted chocolate.