I developed this recipe almost 20 years ago for a Memorial Day party, I was looking for something different to do with rhubarb and hit on this idea. I've made it almost every spring since then. Rhubarb imparts a wonderful tanginess to this ethereal French concoction of air and sweetened cream. For that particular event - and a few times since - I made cupped tuiles out of almond flour to serve the mousse in. The almond presents a nice flavor backdrop to the rhubarb. But the tuiles are just gilding the lily.