Beat the butter until frothy. Gradually stir in the sugar, eggs, flour, cornstarch, baking powder, salt, milk and sparkling water. Peel and core the apple and cut into small pieces. Stir the apples, cinnamon and sultanas into the batter. Grease a loaf pan and add the batter. Bake the cake at 175 ° C top-bottom heat on the middle rack for about 50 minutes, then remove and let cool down.
My favorite sponge cake recipe that I often modify - the apple and cinnamon variant, however, occupies the absolute top spot!