Beat the eggs with the sugar, add the oil and yoghurt. Sift the flour with the baking powder and add them to the mixture.
Set aside 1/3 of the mixture, put the cocoa in it. Pour the white mixture into paper muffin forms, pour in a little of the brown mixture over it.
Bake the muffins at 356°F (180 °C) for 20-25 min.