Prepare a 9 x 13-inch baking pan by lightly coating with cooking spray. In a small bowl, whisk together 1/2 cup cornstarch and 1/2 cup powdered sugar. Dust the bottom and sides of the pan with the cornstarch mixture (it helps to use a pastry brush). Tap off the excess and reserve. In a small bowl, sprinkle 3 tablespoons plus 2 teaspoons (or, 1 oz.) unflavored gelatin over the surface of 1/3 cup water. Whisk together briefly, and set aside to soften. In a medium saucepan, combine 2 cups granulated sugar, 1/2 cup rhubarb syrup (from the recipe above), and 1/2 cup water. Clip a candy thermometer to the side of the pan, and over medium-low heat, cook the mixture to 246 (F). As the mixture approaches 246 (F), it will start to creep up the sides of the pan — simply whisk or remove from heat for a few seconds to prevent the mixture from boiling over.
Meanwhile, in the bowl of a stand mixer fitted with a whisk attachment, combine 3 egg whites, 1/2 tablespoon vanilla extract, 1/4 teaspoon kosher salt, and a pinch of cream of tartar. When the syrup (from the third step) reaches 230 (F), start to whisk the egg white mixture in the mixer's lowest speed.
Once the syrup mixture reaches 246 (F), immediately remove the syrup from heat and whisk in the softened gelatin (from the second step) until no lumps remain. Pour the syrup through a fine-mesh sieve over a heatproof bowl or large liquid measuring cup.
With the mixer still on low speed, pour in a small amount of syrup away from the whisk so that the hot syrup doesn't splash. Continue to add the syrup in a thin stream; when all the syrup has been added, raise the speed to medium-high. Continue to whisk until the meringue has cooled to room temperature and stiff peaks form — this should take about 15 to 20 minutes, depending on your mixer.
Once stiff peaks have formed in your meringue, use a rubber spatula to scrape the meringue out into your prepared 9 x 13-inch baking pan and smooth the top. Let rest for an hour uncovered, before covering the pan and allowing the marshmallows to set for approximately 6 hours.
Once the marshmallows have set, invert the slab onto a cutting board dusted lightly with the remaining cornstarch and powdered sugar mixture (from the first step). Use a lightly oiled serrated knife to cut the marshmallows into 1 1/2-inch squares Dust the cut edges with the remaining cornstarch mixture and store in an airtight container or bag for up to 5 days.