Look for the ripest and plumpest grapes you can find to yield a sweeter and more vibrant-purple dessert. For an even more elegant presentation, unmold the jellies: Run a knife carefully around the edge and warm the bottom of each ramekin in a hot-water bath before inverting it onto a dessert plate (shake the jelly loose). If you have graham crackers on hand, crush and sprinkle them on top of the whipped cream.
Combine the grapes, sugar, 1/4 cup heavy cream and 1/2 cup water in a medium saucepan over medium heat. Cook, partially covered, stirring occasionally, until the grapes are soft and the sugar has melted, about 10 minutes. Test one of the grapes to see if it is soft enough by pressing it against the side of the pot: If it smashes easily, it is ready; set aside to steep for 5 minutes.
Put 1/2 cup cold water in a medium bowl and sprinkle the gelatin on top. Let sit until the gelatin is fully absorbed (Do not stir.) Strain the hot grape mixture into the gelatin, whisking gently. Press on the solids in the strainer to extract as much liquid as possible. Discard solids. You should have 3 cups liquid. Skim foam if desired.
Pour the mixture into six 4-ounce jelly jars, ramekins or pudding cups. Chill until set, at least 2 hours and up to overnight.
Before serving, whip the remaining 1/3 cup heavy cream to soft peaks with a hand mixer. Dollop the cream on top of the jellies and sprinkle with the peanuts.
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