Directions
For the best results, all ingredients should be at room temperature. In a heavy medium saucepan over low heat, melt the chocolate with the water, stirring until smooth and shiny. (The mixture must never be hot, only warm to the touch.)
Remove from the heat. Stir in the butter and light sour cream slowly until smooth. Whisk each yolk into the chocolate mixture, one at a time, gently and slowly so as not to stiffen the chocolate, but still to maintain the gloss.
In the bowl of an electric mixer, beat the whites with the salt until foamy. Add the sugar and beat until stiff but not dry. Scrape the chocolate mixture into a medium bowl. Stir a small amount of the whites into the chocolate. Fold in the remaining whites until almost smooth.
Scrape into a serving bowl, cover loosely and refrigerate, preferably overnight. Mousse is best chilled for 24 hours. Feel free to decorate the mousse cups with chocolate sprinkles or chips, chopped nuts or a sweet touch of whipped cream.