Add the rice, 250ml of water and the salt and butter to a large saucepan, then cover with a lid and place over a high heat. Bring to the boil then reduce the heat and simmer for 10 minutes until the water has been absorbed and the rice is cooked.
Pour in the milk and add the vanilla sugar. Stir through to combine then bring to the boil again. Reduce the heat and simmer slowly for 25-35 minutes until you have a creamy consistency but the rice still has a little bite to it.
Pour the rice pudding into a baking dish and allow to cool before covering and popping it in the fridge until chilled.
In the meantime, cut the rhubarb into 6cm pieces. Place the sugar, orange zest and juice in a cooking pot with 100ml water, place over a medium-high heat and bring to a steady simmer, stirring until the sugar is dissolved. Add the rhubarb to the pot and poach until it is soft when pierced with a fork. Remove with a slotted spoon and set aside on a plate.
Continue to cook the poaching liquid until it becomes syrupy. Remove from the heat and allow to cool slightly before adding the rhubarb back in.
Serve the chilled pudding with the rhubarb syrup on top.