Clean, peel, wash and chop the celery. Heat butter. Sauté the celery and chestnuts in it. Season with salt and pepper. Sprinkle with 1 tsp. sugar, caramelise slightly. Deglaze with wine.
Add stock and aniseed. Bring to the boil and simmer for about 25 minutes until the chestnuts are soft.
Puree the soup with a hand blender and pour on the cream. Bring to the boil and pass through a sieve into a pot.
Rinse mussels cold and dab dry. Heat the oil in a pan. Fry the mussels for 2-3 minutes, turning them once - they should still be slightly shiny in the middle. Season with salt and pepper.
Bring the soup to the boil again. Season to taste with gin, salt and cayenne pepper. Serve with the mussels. Garnish with marjoram as desired.