Whisk the egg well. Dissolve the baking soda in the yoghurt and add it to the egg.
While continuously whipping, add the oil, sugar, cinnamon and cocoa. Gradually add the sifted flour and stir it with a wooden spoon until the mixture becomes homogeneous.
Finally, add the jam and mix. Pour the cake mixture carefully into a baking tray laid out with baking paper. Put it to bake in a preheated oven, at 374°F (190 °C).
Check for readiness with a toothpick, which needs to come out without crumbs on it. Serve the cocoa cake with jam completely cooled and cut into pieces.