Preheat oven to 170C (340F) temperature. Spread butter on baking tray.
Then mix flour with cocoa powder, baking powder and salt. Boil the cream and add to it chopped chocolate, stir while chocolate will melt. Add kefir, stir and set aside to rest. Whisk butter with sugar until it becomes fluffy. Add egg and 1 egg white. Add vanilla extract. Mix everything very well. Then in batter, add a little bit of the flour mixture, mix to combine. Then add a little bit of chocolate, and again, mix very well. Repeat while you run out of flour and chocolate mixtures. Then pour the batter into baking tray and bake for about an hour.
For icing: boil up the cream. Then turn off the heating. Add butter and crushed chocolate. Pour syrup. Mix very well to combine and set aside. When cake is done, pour icing on it.