Mix the butter with sugar, vanilla sugar, the zest of half a lemon and salt until frothy. Stir in the eggs. Mix and stir in flour, hazelnuts and baking powder. Cut the banana and kiwi into small cubes and fold in.
Fill the dough into a greased muffin tin with 12 hollows and bake in the hot oven for about 25 minutes.
Let the muffins cool for 10 minutes, then remove them from the mold and let them cool on a wire rack. Decorate as you wish.