1 Cook the rhubarb: Place a medium-sized, heat-proof bowl on the counter next to the stove. In a wide, deep skillet over medium heat, stir together the sugar, orange zest and orange juice. Stir constantly until the mixture liquefies and begins to boil. (It’s fine if not all the sugar is dissolved at this point.)
Raise the heat to high and add the rhubarb. Cook, stirring constantly, until the rhubarb juices start to flow, the liquid comes to a boil, and the sugar dissolves completely.
Adjust the heat to maintain a gentle simmer. Cook, stirring often, for an additional 4 to 6 minutes, or until the rhubarb starts to fall apart, but about one-third of the chunks remain intact. Taste and add more sugar, 1 tablespoon at a time, if you like.
2 Cool the rhubarb: Immediately pour the rhubarb puree into the heatproof bowl and stir in the rosewater, if using. Cool to room temperature; the rhubarb will continue to soften as it sits.
3 Refrigerate the rhubarb for 2 to 3 hours, or until completely chilled. This rhubarb mixture can also be prepared several days in advance and kept refrigerated, or frozen for up to 3 months. (Thaw in the fridge overnight, if frozen.)
4 Whip the cream: In a chilled bowl, whip the cream with an hand mixer or stand mixer until it forms soft peaks but is slightly under-whipped. Whisk in the sour cream just until combined (the cream should still form soft peaks).
4 Assemble the fool: Add the whipped cream to the bowl of rhubarb. With a rubber spatula, swirl the cream and rhubarb together so that the mixture has streaks of rhubarb, but is not fully mixed. Spoon the fool into tall dessert dishes or wine glasses, and chill until ready to serve.
The fool is best eaten on the day it is made, but leftovers will keep, covered, for a few days.