Press juice of the berries, mix the pulp with water, bring to boil and strain the broth in a separate bowl. To the broth add sugar, cool, add the soaked gelatin and bring the mixture to boil, stirring, mix with the buckthorn juice and whisk until foamy mass.

Pour the mousse in molds and refrigerate to congeal.

Before serving, plunge the mold in hot water for several seconds and turn the mousse on a plate.