Mix butter with sugar and 1 egg until foamy, then alternately stir in the flour and cornstarch mixed with baking powder and the second egg. Pour the dough into a greased springform pan, cover with cut apple halves, the curved side up. Bake the cake for 10 minutes at 180 ° (preheated), then place the halved walnuts on top, also with the curved side up. Bake the cake in 30 minutes at 180 ° in the preheated oven.