I’m not sure how vanilla would work. The temperature of the mixture may cause it to have an “off” taste. For sure, if you try it, I’d recommend only using real vanilla, not imitation.

I’ve never tried vanilla, Debbie. You may want to add a pinch of salt if you are using unsalted butter. The salt will help stabilize the mixture, keeping the water from evaporating too quickly, which can lead to the butter and sugar separating.