For the Drink:
(1) Splash the tiniest bit of rhubarb syrup into the bottom of a metal julep cup. Using your (clean) fingers, gently press the mint sprigs into the bottom and sides of the cup (the syrup will coat the mint leaves and help spread their oils).
(2) Fill the cup completely with crushed ice. Pour the bourbon evenly over the ice. Pour the remaining syrup over the bourbon soaked ice.
(3) Top with additional crushed ice, using your clean hands to pack a snow cone-like mound on top. Serve with straw. Optional: top with powdered sugar, chopped rhubarb matchsticks, and additional mint.
For the Syrup:
(1) Combine the chopped rhubarb in a saucepan with water, sugar, and split vanilla bean. Bring to a boil over medium-high heat, stirring occasionally, then lower heat and cover. Let simmer gently for 5 minutes, then remove from heat.
(2) Stir in the orange blossom water, replace lid, and let cool to room temp. Refrigerate overnight, then strain with a nut milk bag or a cheesecloth-covered sieve. Reserve the strained, broken down rhubarb for use as a compote, and store both syrup and compote in the fridge.
[TIP: you can make the syrup without the orange blossom water -- it will still be awesome.]