Bring 1 ½ cups of water to a boil and then remove from heat. Add the hibiscus leaves, cover the pot, and let steep for 10-20 minutes. Strain the hibiscus and you should have about 1 ¼ cups of deeply hued hibiscus-infused water. Set aside to cool to room temperature.
Meanwhile, prep a glass baking dish with cooking spray. Mix together the powdered sugar and cornstarch and then heavily dust the dish with the mixture. Make sure to coat the bottom and the sides of the dish. Do this before proceeding with the recipe because the following steps go fast.
In a large bowl, pour out half of the hibiscus water (3/4 cup plus 2 Tbsp) and add the vanilla and gelatin, stirring to combine. Have this bowl ready and set aside.
In a medium-sized saucepan, combine the sugar, corn syrup, salt, and other half of the hibiscus water. Using a candy thermometer, heat the mixture to 240F, stirring constantly. Be very careful during this step, as the mixture will begin to bubble quite suddenly when it is ready. When the mixture reaches 240F, immediately begin pouring it into the bowl with the gelatin mixture. Beginning on a slow speed and gradually increasing, use either a stand mixer with whisk attachments or a hand mixer to combine as you slowly pour the hot sugar mixture in. Mix for about 10 minutes. The mixture should become fluffy.
Pour the mixture into the prepped baking dish and sprinkle with additional powdered sugar. Let set undisturbed to firm up overnight. Cut either into squares or with cookie cutters to your desired shape.