Beat the eggs with sugar and oil. Once you obtain a smooth cream, add the sifted flour. Stir the baking soda into the yoghurt and add it to the mixture. Then add the vanilla, cocoa, sour cherry jam, poppy seeds, and optionally - crushed walnuts.
Transfer the mixture into a greased and floured mold and put it to bake in a moderately hot oven at about 320°F (160 °C). Towards the end, poke it with a stick to see if it is ready - it needs to come out dry.
Serve the sponge cake with sour cherry jam sliced and garnished with a little more sour cherry jam.