Preheat the oven to 425°F. Trace a 9" circle on a piece of parchment. Flip the paper over and place it on a baking sheet., To make the pastry: Combine the water, butter, and salt in a medium saucepan. Bring to a rolling boil. Remove from the heat and add the flour all at once, stirring vigorously until the mixture comes together in a ball. If necessary, return the thick batter to the heat for a few minutes to help it come together., Remove from the heat and transfer the batter to the bowl of a stand mixer fitted with a paddle. Set a timer for 7 minutes; you want to cool the mixture, but not too far. When the timer goes off, check the temperature of the batter; you should be able to hold a finger in it for a few seconds. If you have a digital thermometer, the temperature should be below 125°F., Beat in the eggs one at a time, mixing until each egg is absorbed before adding the next. Beat well after adding the last egg; the batter should be smooth., Transfer the batter to a pastry bag fitted with a large star tip. Pipe in two concentric circles over the circle on the parchment; pipe a third circle on top of the bottom two., Bake for 20 minutes, until the pastry begins to puff. Reduce the oven temperature to 375°F and bake for another 30 minutes, until deep golden brown. Remove from the oven and pierce the top with a paring knife in several places. Return to the oven, turn the oven off, and leave in the oven as it cools for 15 minutes. Remove and finish cooling on a rack., To make the filling: Place the rhubarb and 1/4 cup of the granulated sugar in a small saucepan and cook over medium heat until the rhubarb collapses. Continue cooking, stirring frequently, until the mixture thickens slightly, about 5 minutes. Remove from the heat. Stir the remaining 1/4 cup sugar and 1 tablespoon of the ClearJel together, and whisk the mixture into the cooked rhubarb. Chill until ready to use., In a large mixing bowl, whip the heavy cream until soft peaks form. Combine the confectioners' sugar with the remaining tablespoon of Instant ClearJel and add to the whipped cream. Beat the cream until it's light and fluffy, and holds a stiff peak. Stir in the vanilla., To assemble: When the pastry is cool, split it horizontally; set the top aside. Spread the rhubarb filling around the inside of the base. Pipe the whipped cream decoratively on top; it should be about 1 1/2" to 2" high. Replace the top of the pastry, and dust with confectioners' sugar before serving., Store any leftovers in the refrigerator; pastry is best served the same day.