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Boiled Frosting
1 cup sugar 4 tablespoons water 1 egg white 1 teaspoon vanilla Boil the sugar and water until the syrup will thread when dropped from the tip of a spoon. Pour gradually over the stiffly beaten white, beating constantly until the mixture is of the right consistency to spread; then add flavoring and spread evenly with a knife over the cold cake.
This is an Italian meringue in all but name: hot syrup beaten into egg white until it holds a spreadable gloss. The whole recipe hinges on the syrup temperature, which the original describes only by eye.
- The thread test in numbersSyrup that "threads" from a spoon is roughly 230-235F on a candy thermometer. Take it to 238-240F (soft ball) if you want a frosting that sets firm enough to slice cleanly; much below 230F it is likely to stay soft and slow to stiffen.
- Pour, do not dumpAdd the syrup in a thin steady stream down the inside of the bowl while the mixer runs, avoiding the whisk. Syrup landing on the beater flings into hard threads and is lost to the frosting.
- Keep the syrup clearStir only until the sugar dissolves, then leave it alone and wash any crystals from the pan sides with a wet brush. Stirred or seeded syrup crystallizes and the finished frosting turns grainy.
- Beat until cool"The right consistency to spread" means the bowl no longer feels warm and the meringue holds a peak that just bends at the tip. Expect 6 to 10 minutes with a stand mixer, considerably longer by hand, which is what this recipe assumed.
- Egg white quantityOne large white is about 2 tablespoons, which is what this proportion of syrup expects.
- YieldRoughly enough to cover the top and sides of one 8- or 9-inch single layer. Double it for a two-layer cake.
- Humidity mattersBoiled frosting is a sugar syrup and draws water from the air. On a damp day it stays tacky and may weep; make it the day it is served and leave the cake uncovered rather than sealed in a box.
- Flavoring lastVanilla goes in at the end, off the heat, as directed. Added to the boiling syrup the alcohol carries the flavor off with the steam.
History: Boiled frosting, related to the "white mountain" and (by a different method, beaten over hot water) "seven-minute" frostings, was a common American cake covering in the period. A hot-syrup meringue spread smoothly, held a peak, and dried to a thin crisp shell. The technique is the same one confectioners used for meringue and marshmallow work, and it reached the domestic kitchen through the cookbooks and cooking schools of the late nineteenth and early twentieth centuries. That the instruction here is "boil until the syrup will thread" rather than a temperature is telling: thermometers existed but were not yet assumed, and cooks were trained to read sugar by its behaviour on a spoon.