For the crust: Combine the pistachios, sugar and salt in a food processor and blend to a coarse crumb. Add the coconut oil and pulse until the mixture crumbles together. Distribute the mixture among 8 serving glasses and use a muddler or the back of a spoon to press it down evenly and firmly. Set aside.
For the malabi: Whisk together the cornstarch, vanilla bean paste, coconut extract, rosewater, if using, and 1/2 cup coconut milk in a medium bowl.
Whisk together the sugar, salt and the remaining coconut milk in a medium saucepan over medium heat. Cook, whisking often, until simmering, then pour in the cornstarch mixture in a slow, steady stream, whisking constantly. Continue to cook, whisking, until the mixture has thickened, 1 to 2 minutes. Taste and adjust the extracts and flavorings as desired. Divide the mixture among the 8 serving glasses (about 1/2 cup in each), then cover and chill for 4 hours or up to overnight.
For the rhubarb: Preheat the oven to 325 degrees F.
Spread the rhubarb on a rimmed baking sheet and top it with the sugar, vanilla bean paste, salt and lemon zest. Cover with foil and roast for 20 minutes, then remove the foil and roast for another 15 minutes. Let cool to room temperature. (Store overnight refrigerated in an airtight container if making this the day before.) Spoon directly on top of the malabi before serving.