1. In a medium saucepan, combine the rhubarb with the water and sugar. Place over medium-high heat and bring to a boil. Once the mixture begins to boil, turn the heat to low and simmer until the rhubarb is softened, 12 to 15 minutes. Use a slotted spoon to remove the softened stalks from the syrup; set the stalks aside. Pour the syrup into a container and refrigerate.

2. Make the custard: In a small saucepan set over medium-high heat, add 2½ of the cream and vanilla bean. Just before the mixture begins to boil, remove from the heat and set aside for 10 minutes.

3. In a medium bowl, whisk together the egg yolks and sugar until the yolks turn pale in color, about 5 minutes. Carefully pour the warm cream over the eggs; discard the vanilla bean and stir to combine. Pour the mixture back into the pan and place over medium-low heat, stirring with a wooden spoon until the custard begins to thicken, about 5 minutes. Strain the custard through a fine-mesh sieve set over a bowl. Cover the bowl with plastic wrap and refrigerate until cool, about 1 hour.

4. In a medium bowl, whisk the remaining 1 cup of cream until it forms soft peaks. Use a rubber spatula to gently fold 1 cup of the cooled custard into the whipped cream; reserve the rest of the custard for another use. Add the cooked rhubarb and fold in gently, so that the fruit forms pale pink streaks through the custard and cream. Refrigerate for 30 minutes, then spoon into glasses and serve, pouring a little of the reserved rhubarb syrup over each portion.