I highly recommend using corn syrup and not cream of tartar. It turns out yummy with corn syrup. The texture is fabulous too. I've heard that the cream of tartar gives it a weird aftertaste.
This is almost exactly the same as the Betty Crocker Double Boiler (7-minute) Frosting (1970 edition), which does not include the salt and has only 1 tsp. of vanilla. My mother used this on all of our birthday cakes when we were kids ... many fond memories!