FOR THE RHUBARB SYRUP
Combine the diced rhubarb, sugar, vanilla bean, kosher salt, and orange juice in a small saucepan and place over medium-high heat.
Using a silicone spatula, stir often and allow the mixture to simmer and come to a boil, about 3-4 minutes.
Once the mixture reaches a boil, turn the heat down to medium-low and simmer for 20-25 minutes, stirring occasionally until the rhubarb has broken down and the liquid has reduced by about 1/3. The consistency should be slightly thick and syrupy.
Strain the syrup into a clean jar through a fine-mesh sieve, and allow to come to room temperature before covering and transferring to the fridge.
The syrup will keep, stored in an airtight container in the fridge for over a month.
FOR THE RHUBARB ELDERFLOWER ICE CREAM FLOAT
Pour the syrup in the bottom of a glass, and add the sparkling water.
Using a spoon, stir gently to combine.
Add the elderflower liqueur, and stir to combine.
Add a scoop (or two!) of good quality vanilla ice cream to the glass, and serve with a tall spoon and straw.
The rhubarb elderflower soda recipe can be doubled, tripled, quadrupled, and so forth quite easily. If serving the floats for a crowd, you can make the soda a few hours ahead of time and store it in the fridge prior to serving.
The syrup may settle a bit at the bottom of the bottle or bowl when you store it in the fridge. So give it a gentle stir or shake before pouring into glasses, topping with ice cream, and serving.