How to cook
In a suitable bowl measure out the yoghurt and dissolve the baking soda in it. Wait about 5 minutes to let it dissolve thoroughly, add the sugar, stir again and wait the same amount of time. Add the melted butter while stirring gently.
Sift the flour with the cocoa and finally add it to the mixture. Stir until you get a homogeneous blend. Preheat the bottom element of the oven in advance. Grease the bottom and sides of a cake form with butter and sprinkle with flour.
Pour half of the chocolate mixture in the form. On top of it, add the jam evenly and then pour on the second half of the chocolate mixture. Sprinkle chopped walnuts on the very top and place in the oven to bake.
Bake for the first 10 minutes on the bottom heating element and then at 392°F (200 °C) until ready. Poke with a toothpick and if it comes out dry, the cake is ready.
Allow it to cool and in the meantime, heat the liquid chocolate in a water bath. Use it to cover the cake (you may cover all of it, or you may pour the chocolate around the walnuts). Store in the refrigerator until it's time to serve.