Preheat the oven to 200°C (gas 6). Line the base of a 25 cm springform cake tin with baking parchment, then lightly grease and flour the sides.
Beat together the softened butter, sugar, vanilla sugar and salt until creamy. Add the eggs, one at a time, and beat well to make a smooth batter.
Mix the flour with the baking powder and sieve into the mixture. Add the walnuts and fold in alternately with the milk until combined. Pour into the tin and spread level.
Wash the persimmons, pat dry and cut into 2-3 cm wide wedges, removing the stalk. Tuck the fruit wedges evenly into the cake mix.
Bake for 40-45 minutes until lightly browned and a cocktail stick inserted into the mixture comes out clean.
Allow to cool slightly, remove from the tin and leave to cool completely on a wire rack. Sprinkle with cocoa nibs to serve.