Cut 2 bananas into large pieces. Place in a bowl, add lemon juice and vanilla sugar. Puree with the cutting stick of the hand mixer. Add yoghurt and hazelnuts and mix well.
Pour into a metal bowl and leave to set in the freezer for 2-3 hours. Stir well with a whisk about every 1/2 hour to keep the mixture creamy. Cut the nectarine into slices and the rest of the banana into slices.
Form ice cream into balls and arrange on plates with the fruit. Sprinkle with cinnamon and hazelnut leaves. Decorate with lemon balm.