In a separate container, mix the kefir with the baking soda and let stand for about 10 minutes.
Meanwhile, beat the eggs with the sugar for 7-10 minutes, until fluffy. Add the kefir-soda mixture and the vegetable oil, and beat again.
Sift the flour, baking powder, and cocoa into the batter, and mix with a spatula.
Peel the apples, cut into pieces, add to the batter, and fold in with a spatula. Pour the batter into a greased pan and level it out.
For the filling, mix the cottage cheese, sour cream, egg, and sugar in a bowl, then beat with a mixer into a creamy mixture.
Spoon the cottage cheese mixture around the perimeter of the pan over the batter, then use the same spoon to swirl it into the batter - but don't overdo it.
Bake in a preheated 180°C oven for 35-40 minutes. Check doneness with a toothpick - it should come out dry. Let the cake cool, then dig in. Enjoy!