Beat the eggs with the sugar and add the yoghurt with the baking soda dissolved in it. Pour in the oil, lemon zest, vanilla and gradually the flour with hartshorn. You need to get a smooth and soft, non-sticky dough.
Roll it out on a floured surface to about a 1/2″ (1 cm) thickness and cut out cookies of different shapes - bunnies, butterflies, etc.
Arrange them on a tray with baking paper, brush them with beaten egg yolk and sprinkle with sugar. Put them to bake in a preheated 356°F (180 °C) oven for about 15 minutes.