Sprinkle the gelatin evenly over the puree. Stir and let sit for about 5 minutes.
Heat the puree and gelatin mixture over medium-low heat, stirring often or swirling the pan, until the gelatin has all melted and the mixture is completely smooth and not grainy, about 5 minutes. Do not let it come to a boil.
Strain through a fine mesh strainer. The mixture should not be very hot to begin with, but make sure it cools to at least 90F before proceeding.
Whip the cream and salt on medium speed, adding in the sugar a bit at a time, until you've reached medium peaks.
Scrape the bowl and whip in the cooled fruit puree.
Scrape the bowl again and whip to make sure the mixture is blended.
Pour or pipe into dessert dishes (for mousse) or use to decorate your favorite cakes/cupcakes.
Notes
This frosting needs to stay refrigerated, so it will work best on cake/s that have no solid fat in them that will get harder in the fridge. Angel food or chiffon cakes come to mind. Any cake that is either fat-free or uses liquid fat in the recipe should be just fine.To make stabilized whipped cream without the fruity goodness, just leave out the puree and bloom the gelatin (1/2 teaspoon per cup, as before) in 1-2 Tablespoons of cold water. Heat to melt the gelatin and then whisk into your cream after you've sweetened it. Make sure you are whisking/whipping madly to prevent any weird gelatin nuggets from messing up your beautiful cream.