Beat the eggs with the sugar and vanilla. To them, add the oil and stir well.
Next, pour in the yoghurt with the baking soda and finally the sifted flour. Knead a soft dough, add more flour if needed.
Leave the dough in the fridge for about 30 min. to make it easier to form balls the size of a walnut later.
Arrange the cookies in a tray onto baking paper. Smear them with the beaten yolk and sprinkle with sugar. Bake in a moderate oven until reddened.