Yellow Frosting

A two-ingredient uncooked frosting of the sort that filled the margins of 1870s receipt books. The reason it cannot be recommended is the reason the original author already half-admits in her last sentence.

• Raw yolk, never heatedThe yolk is simply beaten cold into the sugar; nothing in the process reaches a temperature that would kill Salmonella. Unlike a boiled or meringue frosting there is no hot syrup poured in, and unlike a mayonnaise there is no acid; sugar alone at this ratio does not make the yolk safe.
• "Use the same day it is made"The instruction may simply reflect that the mixture does not keep, whether from spoilage or from drying; the original gives no reason. Either way, a frosted cake left out on the sideboard for an afternoon is the sort of warm, unrefrigerated holding time that makes raw egg a problem. If you want a yellow frosting, use a current recipe built on a cooked base.
• Pulverized sugarThis is the nineteenth-century name for finely ground sugar, a forerunner of confectioners' sugar. It carried no cornstarch and was generally coarser than modern 10X powdered sugar, so a modern substitution by volume will not behave identically.
• "Nine heaping tea-spoons" is not a fixed amountA heaping spoon of powdered sugar varies enormously with how it is scooped, and period teaspoons were not standardised at the modern 5 ml measure. There is no honest single conversion here, though the recipe does give a stated ratio rather than leaving it to the eye.
• YieldOne yolk with that much sugar makes only a few tablespoons of frosting, far too little to fill and cover a layer cake.
• FlavoringThe recipe gives no quantity of vanilla at all. Extracts of the period varied widely in strength, and adulteration of "vanilla" flavorings with coumarin-based tonka substitutes was a known problem; coumarin is no longer permitted in food.