Gently warm the heavy cream and add the vanilla bean, letting it steep.
Boil the chunks of parsnips in salted water. When very tender, drain and either put the parsnips in a blender, or back into the pot if you are using a stick blender.
Scrape the vanilla bean into the heavy cream and slowly add to the parsnips as you blend. Add the butter. Puree until you have a velvety smooth texture. Taste and add some more salt if necessary.
Here is a link for everything you want to know about vanilla beans.