Mix flour (1.5 cups) with baking powder, add butter and rub until crumbly. Pour 0.5 cup sour cream and start kneading dough. The dough may seem too wet and sticky, but try to accumulate it and knead for a few minutes. The dough will amass and become soft and hardly sticky. Otherwise, you can add a handful or two more flour. Oil the baking dish, put the dough in it and smooth with the hands on the bottom and along the edges. Peel apples, remove the core and cut thin. Put it in the baking dish. Slightly whip 1 cup sour cream, egg, sugar and 2 tablespoons flour in a separate bowl. You’ll get a rather liquid mixture. You can optionally add cinnamon or vanilla to it. Evenly cover the apple stuffing with a sour cream mixture. Place the baking dish in the oven preheated to 200 °C and bake for about 50 minutes until golden brown (you should be guided by the dough, as the filling may remain unset: it will set when cooled). Cool the cake to room temperature and refrigerate for 6-12 hours.