Mix the yoghurt and sour cream until smooth until there are no lumps. Add the baking soda and leave for 2-3 minutes to bubble. Add the two types of sugar and stir with a wooden spoon without a mixer.
Add the flour in portions, mix the last one with the walnuts and pour into the mixture. This is done to prevent them from burning in the dough and falling to the bottom. Mix until all the dry ingredients are absorbed. Add the oil in portions, stirring until each portion is absorbed by the dough before adding the next. Finally, put the essence in.
Smear a baking tray (diameter 11″ (28 cm)) with butter and put half the mixture into it. Place in the oven, bake at 356°F (180 °C). While baking, dilute the other half of the cake batter with the milk. If the jam is too thick, it can be diluted with a little water, so it's easier to distribute it along the baked cake layer.
Once the layer is baked, put jam evenly on it. On top of the jam, put the second half of the dough, return it to the oven and bake at 392°F (200 °C) on the top element only. When ready, remove the cake from the oven, allow it to rest for 5 minutes, then add chocolate icing and sprinkle it with coconut flakes.
Of course you can prepare a topping according to your own taste and preference.