Wash the apple, quarter it, cut out the core. Coarsely grate the fruit flesh. Peel banana, cut into thin slices. Sprinkle fruit with lemon juice. Beat butter and sugar until creamy. Stir in eggs one by one, add flavour.
Mix flour and baking powder. Alternate with the smoothie and stir in briefly, fold in fruit. Pour into a greased, breadcrumbed ring cake mould (2 1/4 litre capacity). Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 70 minutes. Let it cool down a little bit, turn out of the mould and let it cool down completely. Dust with icing sugar