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Boiled Frosting
2 cups sugar. 1/2 cup water.
Whites 2 eggs. 1/4 teaspoon cream of tartar.
PROCESS: Mix sugar, cream of tartar and water in a sauce pan. Place on range and stir until mixture begins to boil. When syrup drops from the wooden spoon thick like honey, remove from range and add eight tablespoons of the syrup to the stiffly beaten whites of eggs, beating constantly. Return remaining syrup to range, continue cooking until syrup spins a thread at least five inches in length. Pour syrup in a thin stream onto first mixture and beat until cool and slightly glazed on side of bowl. Spread thickly on cake.
A classic boiled icing — what a pastry kitchen would now call an Italian meringue. The syrup temperature is the whole recipe; everything else is beating.
- The sugar is missingThe ingredient list gives water, egg whites and cream of tartar but no quantity of sugar, though the process calls for it. Comparable boiled frostings run somewhere around 1 1/2 to 2 cups granulated sugar to 1/2 cup water and two whites; treat the printed list as incomplete rather than as a proportion to follow.
- "Thick like honey"This is the first, softer stage, somewhere in the region of 225-230F if you are working with a thermometer, though the book's cue is purely visual. The eight tablespoons added here warm and loosen the whites before the hotter syrup goes in.
- "Spins a thread five inches"The thread test is conventionally about 230-235F; a thread as long as five inches implies the upper end of that range, running into the soft-ball stage at roughly 235-240F. A thermometer is more reliable than watching a spoon, and the temperature is what determines whether the frosting sets or stays syrupy.
- Cream of tartarIt inverts part of the sugar so the syrup resists crystallising, which is why the icing stays smooth and spreadable rather than turning grainy. Do not stir the syrup once it boils, and wash any crystals off the pan sides with a wet brush.
- Pour slowly, down the sideAdd the hot syrup in a thin stream against the inside of the bowl while beating, not onto the beaters, or it will spatter into hard sugar shards.
- Beat until coolThe recipe's cue — glazed on the side of the bowl and cool to the touch — is accurate and takes 6-10 minutes in a stand mixer. Stopping while it is still warm gives frosting that slides off the cake.
- Yield and timingWith the sugar quantity absent it is guesswork, but a batch on these lines should cover a two-layer 8- or 9-inch cake. Use it within minutes of finishing; it crusts on the surface and stiffens in the bowl, and it does not keep or rewhip.
- Humid daysBoiled frosting draws moisture from the air and will weep in damp weather. Cooking the syrup a degree or two higher helps; refrigerating a finished cake does not.
Boiled frosting was the standard American cake covering before butter-heavy buttercreams took over in the mid-twentieth century: sugar syrup beaten into egg whites, giving a glossy white coat that firmed to a thin crust over a marshmallowy interior. It is the same construction a modern pastry cook calls Italian meringue, and the fact that a household cookbook of this era expected a cook to judge syrup by how it dripped from a wooden spoon and how far it would spin a thread suggests kitchens where candy thermometers may not have been common and sugar work was learned by eye. Books like this one gave the frosting no fanfare, and the labour of beating it by hand until cool was assumed.