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Marshmallow Icing
3/4 cup granulated sugar 1/3 cup water 6 or 8 marshmallows white of 1 egg few drops vanilla extract Boil sugar and water without stirring until syrup spins a thread; melt marshmallows in syrup; pour slowly over beaten white of egg; add flavoring and spread very thickly over cake. Melt 2 squares unsweetened chocolate with one-half teaspoon butter and spread thin coating over icing when cool.
A boiled-syrup meringue icing with marshmallows melted into it, finished with a thin chocolate glaze. The whole thing hinges on the syrup: too cool and the icing weeps, too hot and it sets before it reaches the cake.
- "Spins a thread"That is the thread stage, 230-235F on a candy thermometer. From 3/4 cup sugar and 1/3 cup water it takes only a few minutes at a steady boil, so start checking early.
- Do not stirThe instruction is literal. Stirring after the sugar dissolves seeds crystallisation and the icing turns grainy. Wash the sides of the pan down with a wet pastry brush instead.
- The marshmallowsTaking them as equivalent to today's large marshmallows, 6 to 8 works out at roughly 2 ounces, or about a cup of miniatures. Stir them into the hot syrup off the heat and they will slump quickly.
- Beat the white firstHave the egg white at soft peaks before the syrup is ready, then pour the syrup down the side of the bowl in a thin stream while beating. Dumping it in cooks the white into threads.
- Egg safetyThe hot syrup heats but does not reliably pasteurise the white. Anyone serving this to the very young, elderly or immune-compromised should use pasteurised whites or powdered egg white.
- Spread while warmMarshmallow icing sets quickly and does not spread again once skinned over. Work fast, and pile it on thickly as directed rather than trying to smooth it thin.
- 2 squares chocolateA square is 1 ounce, so 2 ounces of unsweetened baking chocolate. The half teaspoon of butter keeps it glossy and pourable; spread it only once the icing beneath is completely cool or it will slide.
- YieldRoughly enough to cover the top and sides of a single 8- or 9-inch layer, or the top of a 9x13 sheet, though the recipe specifies no yield.
The New Royal Cook Book was one of the promotional books put out by the Royal Baking Powder Company, distributed cheaply or free to sell leavening, and its cake section is correspondingly long. Marshmallow icing belongs to the moment when marshmallows were becoming a pantry item rather than a confectioner's specialty: the original sweet was thickened with mucilage from the marsh mallow root, but manufacturers had largely moved to gelatin and corn syrup by around the turn of the twentieth century, and boxed marshmallows were cheap and widely available in American kitchens. The technique underneath is older: boiled sugar syrup beaten into egg white, broadly the method behind Italian meringue, with the marshmallows adding gelatin that helps keep the icing from cracking as it dries.