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Boiled Frosting
One cupful of granulated sugar, one-half cupful of boiling water; let them boil together until it threads from a cold spoon. Have ready the white of an egg beaten to a stiff froth. Pour the syrup into the egg slowly, beating all the time; flavor with the juice of half of a fresh lemon.
This is a boiled or Italian meringue frosting: hot sugar syrup whipped into beaten egg white. Everything depends on the syrup being at the right stage, since it is the only thing cooking the white and the only thing that will set the frosting.
- "Threads from a cold spoon"That is thread stage, roughly 230-235F on a candy thermometer. For a frosting that holds a swirl rather than slumping, take it to 238-240F (soft ball); at the low end of thread stage it stays loose and sticky.
- Proportions and yieldOne cup sugar, half a cup water and one egg white gives a fairly small quantity of frosting - enough for the top of a loaf cake or a thin coat on a single small layer. Double it if you need more.
- Have the white ready firstThe syrup will not wait. Whip the white to stiff, glossy peaks before the sugar comes up to temperature, and have the bowl in place under a mixer if you have one. Hand-beating this with a whisk is possible but wants a second pair of hands.
- PouringAdd the syrup in a thin, steady stream down the side of the bowl with the beater running, not onto the whisk itself, which will fling threads of sugar up the sides and set them hard. Keep beating until the outside of the bowl is only barely warm, five to eight minutes.
- Keep the syrup clearDo not stir the syrup once it boils, and do not scrape crystallised sugar off the pan walls into it. A single seed crystal will turn the whole batch grainy. Washing the walls down with a wet pastry brush early on prevents this.
- The lemonThe juice of half a lemon is for flavour; acid can also help steady a meringue, though added at the end as the recipe directs it cannot affect the boiling syrup. Add it once the mixture has cooled somewhat, so the volatile flavour is not driven off.
- Spread promptlyThis frosting skins over as it cools and becomes unspreadable. Get it onto the cake while it still moves easily, then work it with the back of a spoon or an offset spatula. It sets with a thin, crisp shell over a marshmallowy interior.
- WeatherBoiled frostings fail on humid days: the sugar draws water from the air and the surface stays tacky. If it is muggy, cook the syrup two or three degrees higher.
Boiled frosting - called white mountain frosting, or in pastry terms Italian meringue - is often said to have been the standard American cake covering through the later nineteenth century, before buttercream displaced it. Its appearance in a turn-of-the-century household book depends on two things having become ordinary: cheap refined granulated sugar, which pours and dissolves cleanly where loaf sugar had to be broken and pounded, and the rotary egg beater, which made stiff whites a matter of a minute rather than a chore with a fork. Note that the recipe gives no thermometer reading; cooks were expected to judge the syrup by eye, spooning a little up and watching whether it fell in a thread, and the same test governed candy-making in the period. The lemon juice at the end is characteristic of the era's frostings.