Stir the baking soda with the yoghurt, add the sugar and oil, beat with a mixer. Add the flour and cocoa, continue beating.
Prepare a round cake form with a hole, oil and flour it, pour in the mixture. Bake at 392°F (200 °C) until ready (all ovens are different, so just poke the cake with a toothpick and see if it comes out clean).
After the cake has cooled, smear it with the cream, prepared by stirring the softened butter with the powdered sugar to a homogeneous mixture.
On top you can decorate with grated chocolate, sugar sprinkles or other decorations. I grated a chocolate wafer.