Sweat the shallots in a pan with a little butter and a pinch of salt until translucent. Add the chestnuts and toast briefly. Deglaze with the sweet wine and let it boil down a little.
Pour the beef soup on top and simmer for about 15 minutes. Then stir in the cream and sour cream and simmer for another 5 minutes until the chestnuts are quite soft. Mix the soup finely in a food processor and strain it through a sieve. Season to taste with salt and mix with rum and a nut-sized piece of butter just before serving until a nice foam is formed.