Have ready 4-5 small (4 ounce), heat-proof glass jars or coffee/cappuccino cups or ramekins.
In a medium-sized, heavy-bottomed saucepan, combine the coconut milk and vanilla pod and scrapings. Heat over a medium flame, stirring occasionally, until the milk is hot and steamy (don't let it boil or it may scorch or separate). Cover and let steep 10 minutes or up to 1 hour to infuse with the vanilla.
Meanwhile, whisk together the sugar, cornstarch, and salt in a small bowl. When the coconut milk has steeped, whisk a few tablespoons of it into the cornstarch mixture to make a slurry, then whisk the slurry into the warm coconut milk. Cook the pudding over medium heat, stirring constantly with a heat-proof silicone spatula, until it comes to a boil. Still stirring, boil for 1 minute; it should be the texture of runny yogurt. Immediately strain the haupia through a mesh sieve and into a heatproof measuring pitcher, working it through with the spatula. (You can rinse and dry the vanilla pod and stick it in a jar of alcohol to make extract, or in a jar of sugar. Same with the other half, used below.) Divide the pudding evenly among the jars. Cover and chill until firm, at least 3 hours and up to 3 days.
Make the hibiscus rhubarb:
In a heat-proof measuring pitcher, combine the hibiscus with the boiling water. Let steep 10 minutes.
In a medium saucepan, rub the sugar with the vanilla pod and seeds to distribute the seeds evenly. When the hibiscus has steeped, strain the mixture through a mesh sieve and into the pot. Place the pot over medium heat and bring to a boil, swirling the pot occasionally to dissolve the sugar. When the mixture is at a rolling boil, add the rhubarb. Return the mixture to a simmer, then turn off the heat, cover the pot, and let sit until the rhubarb is tender when you pierce it with the tip of a sharp knife, about 10 minutes. (If the rhubarb is still too firm, repeat the simmering/steeping process once more.)
Gently strain the rhubarb, reserving the syrup and returning it to the pot. Simmer the syrup over medium heat until thick and reduced by half, swirling the pot frequently, about 5 minutes. Place the drained rhubarb in a heat-proof container, pour the hot syrup over it, let cool, and chill until cold, at least 2 hours and up to 3 days.
To serve, divide the rhubarb and syrup among the puddings and serve immediately.
I found hibiscus blossoms (also called Jamaica) in the bulk section of my awesome co-op; they can also be found at Latin-American markets. Alternatively, use a few bags of hibiscus tea.Be sure to make both components a few hours ahead to allow them to chill; both can be prepared up to three days before serving and stored, covered, in the refrigerator.Nutritional values are based on one of four servings.