Trim about 2 inches from each end of the rhubarb and discard. Roughly chop the stalks and add to a small stock pot with 2 cups of the sugar and 1 1/2 cups water. Bring to a boil over high heat, stirring occasionally. Cover, leaving the lid slightly ajar, and boil until the rhubarb mostly falls apart, about 3 minutes, stirring once or twice. Remove from the heat and let steep, covered, for 5 minutes.
Place 1/2 cup water in a medium bowl and sprinkle the gelatin on top. Strain the hot rhubarb mixture through a fine mesh strainer right into the gelatin, whisking gently. Press on the solids in the strainer to get as much liquid as possible from the rhubarb. Discard the solids.
If there is a lot of foam and you prefer a clear surface, skim it with a spoon and discard it. Pour the mixture into eight 4-ounce jelly jars. Chill until set, at least 3 hours or overnight.
When ready to serve, whip the cream, vanilla extract and the remaining 1 teaspoon sugar to very soft peaks and dollop it over the gelatin.