First, make the pavlova roulade base. Position a rack in the center of the oven and preheat the oven to 350°F. Generously spray a jelly roll pan with cooking spray and line the bottom with parchment paper. Spray the parchment, too.
Whisk the eggs to medium peaks. In a stand mixer fitted with the whisk attachment, combine the egg whites, cream of tartar, and salt. Whisk on medium-high speed until medium peaks form, about 3 minutes.
Add the water and sugar, then whisk to stiff peaks. Reduce the mixer to medium-low and drizzle in the water. Slowly sprinkle in the sugar, one heaping spoonful at a time (this may take 1 to 2 minutes). Increase the mixer to medium-high and whisk until stiff, glossy peaks form, about 5 minutes.
Fold the cornstarch, vanilla, and vinegar into the egg whites. Use a rubber spatula to gently fold in the cornstarch, vanilla, and vinegar into the egg whites by hand. Assemble for baking. Scrape the batter into the prepared pan and use an offset spatula to spread it evenly and smooth its top. Bake the pavlova. Bake for 25 minutes, or until the pavlova starts to turn a light caramel color. It will puff considerably as it bakes, but will settle once it is out of the oven. Cool the pavlova completely in its pan on a wire rack while you make the rhubarb filling. Make the rhubarb filling. In a medium saucepan over low heat, combine the rhubarb, sugar, and salt for the rhubarb filling. Cook, stirring gently, until the juices thicken and the rhubarb becomes very tender, 10 to 15 minutes. Scrape into a glass bowl and cool slightly on a wire rack while you make the whipped cream filling. Make the whipped cream filling. In a stand mixer fitted with the whisk attachment, combine the cream, confectioners' sugar, and vanilla for the whipped cream filling. Whisk on medium-high speed until soft peaks form, about 3 minutes. Assemble the rhubarb pavlova roulade. Generously dust a sheet of parchment paper with confectioners' sugar. Invert the pavlova onto the prepared parchment paper and carefully peel off the greased parchment paper the pavlova was baked with. Cover the cake with half the whipped cream, then top with half of the rhubarb. Using the parchment paper to get started, roll the cake, starting at the short end, into as tight a log as you can manage. Transfer the log to a serving platter. Loosely cover with plastic wrap and refrigerate for 1 hour.
Serve and store. Top with the remaining whipped cream and rhubarb and serve chilled, straight from the fridge. The rhubarb pavlova roulade will keep in the refrigerator, on its serving platter under a loose sheet of plastic wrap, for up to 3 days.