The secret to this dessert? Elderflower liqueur, which you can buy at most wine and spirit shops. It adds a delicate floral flavor and aroma to the sabayon, a frothy, warm topping.
Prepare an ice-water bath. Bring water, 2 cups sugar, the vanilla seeds and pod, and lemon zest to a boil in a wide, shallow saucepan. Stir. Reduce heat. Simmer for 10 minutes. Add rhubarb, and simmer gently until just tender, 3 minutes. Transfer to a bowl set in ice-water bath. Let cool completely. Divide among 4 bowls.
Whisk together remaining 2 tablespoons sugar, the egg yolks, and the elderflower liqueur in a double boiler or a heatproof bowl set over a pan of simmering water. Cook, whisking constantly, until sabayon is tripled in volume, frothy, and warm to the touch, 6 to 8 minutes. Serve immediately over rhubarb.