Stir together the butter, sugar, 3 yolks and 1 whole egg well.
Add the yoghurt, baking powder, baking soda and stir once again.
Sift the flour and add it to the resulting mixture.
Bake in a greased tray at 302°F (150 °C).
Sprinkle with walnuts and put it to bake a bit once again.