Crumble the biscuits into fine, little pieces. Butter the bottom of a form with a removable ring.
Pour in the biscuits, sprinkle walnuts.
Wash the apples and cut them into thin slices. Arrange them on top of the biscuits. Beat the eggs with the brown sugar. Add the oil and yoghurt.
Add the flour, sifted with the cinnamon and baking powder. Stir and pour the mixture over the apples.
Bake the cake in a preheated 356°F (180 °C) oven for about 30-35 min.
Leave it to cool and invert it. Sprinkle generously with powdered sugar.