Place the egg whites, sugar, and pinch of salt in a heatproof mixer bowl set over a pot of simmering water. Whisk until sugar dissolves and mixture registers 160 degrees on a candy thermometer. Remove the bowl from the heat, and using a whisk attachment, whisk the mixture on medium speed for 5 minutes. Increase the mixer speed to medium-high, and then whisk until stiff, glossy peaks form, about 6 minutes. Reduce the speed to medium again, and add the butter, 1 piece at a time, whisking well after each addition. When the butter has been completely incorporated, switch to the paddle attachment. With the mixer running on low, add the vanilla, and then the cooled rhubarb puree, and beat until smooth, 3 to 5 minutes. Use immediately, or cover, and refrigerate.