Thinly rub the peel of an orange and add to the milk with the sugar. Heat and dissolve the sugar. Dissolve the soaked gelatin in the hot (not boiling) milk. Remove from heat and stir the sour cream evenly. Squeeze the oranges and pour the juice (about 160 ml are needed) through a fine sieve into the cream mixture. Mix and season with the orange liqueur.
Fill into small dessert molds and leave to set in the refrigerator overnight.