Sauté the shallots with butter lightly in a pan until glassy and add a pinch of salt. Add the chestnuts, briefly continue sautéing, then douse with the sweet wine and allow to stew a bit.

Pour on the beef broth and simmer for ca. 15 minutes. Then stir in the cream and sour cream and simmer for another 5 minutes until the chestnuts are completely soft. Mix the soup to a fine consistency in a food processor and stain through a sieve. Season with salt just before serving, add the rum and a nut-sized chunk of butter and mix until a nice foam has formed.