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Icings and Fillings Boiled Icing
1 cup granulated sugar 1/2 teaspoon Royal Baking Powder 1/2 cup water white of 1 egg 1/2 teaspoon flavoring extract Boil sugar, baking powder and water until syrup spins a thread; pour very slowly into stiffly beaten white of egg and beat until smooth; add flavoring as desired. Spread immediately on cake.
A standard boiled icing, with one oddity: the baking powder. Royal's cookbooks were written to put the product into every recipe possible, and here it stands in for the pinch of cream of tartar that older frosting recipes call for.
- "Spins a thread"The thread stage, generally put at about 230-235F on a candy thermometer, though sources vary. Below that the icing stays syrupy and never sets; much above it and you get a hard, brittle crust that cracks when the cake is cut.
- Why the baking powder is thereRoyal was a cream-of-tartar baking powder, and the usual explanation is that its acid inverts a little of the sugar so the syrup does not crystallize into a grainy frosting; how much free acid survives being paired with the powder's soda is another matter. A scant 1/8 teaspoon of cream of tartar does the same job more directly, as does a teaspoon of corn syrup.
- Pour slowly and off the beaterAdd the syrup in a thin stream down the side of the bowl with the white already beaten to stiff peaks. Dumping it in cooks the egg into threads and the icing turns lumpy.
- Beating timeExpect 5 to 8 minutes of hard beating after the syrup goes in, until the bowl is only just warm and the icing holds a peak that folds over at the tip. A stand mixer makes this trivial; by hand it is real work.
- Egg safetyThe hot syrup raises the temperature of the white substantially as it is beaten in, so this is not simply a raw-egg frosting, though it is not a reliable pasteurization either. Use a clean fresh egg and beat the mixture through rather than stopping early.
- FlavoringHalf a teaspoon of extract is modest by current taste; a full teaspoon of vanilla is usual now. Add it at the end, once the icing has cooled, or the alcohol carries the aroma off.
- Work fast, and watch the weatherBoiled icing begins to set as soon as it stops moving, so spread it the moment it is ready. On a humid day it stays tacky and may weep; a dry kitchen gives the characteristic matte, marshmallow crust.
- YieldThe recipe gives no yield; roughly enough to fill and thinly ice one two-layer 8-inch cake, or to top a 9-inch square. Double it for generous swirls.
The New Royal Cook Book was one of the many free booklets issued by the Royal Baking Powder Company in the early twentieth century, and its purpose was as much advertising as instruction. Boiled icing itself is older than baking powder: it belongs to the nineteenth-century sugar-boiling tradition, in which a cook judged syrup by eye and finger — thread, soft ball, hard crack — long before thermometers were common in home kitchens. The same method beaten over hot water rather than with a poured syrup became known as seven-minute frosting, and both were the standard finish for white layer cakes and angel food in American households until buttercream and, later, cream cheese icings displaced them.