Mash the banana and mix with the yogurt. Cut the apple and kiwi fruit into small pieces and stir in. Add some freshly squeezed lemon juice and vanilla sugar. Let it steep until the flavor of the ingredients has fully developed.
Then season to taste and, if necessary, refine by adding lemon juice / vanilla sugar.
Variation: stir in a little effervescent powder with the vanilla sugar, choose the fruit aroma as you like.