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Caramel Filling for Cakes
Three cupfuls of brown sugar, three-quarters cupful of cream, three-quarters of a cupful of butter, boil together until it hardens in water. Beat all the time.
This is a boiled caramel frosting in its simplest form: sugar, cream, butter, cooked to a candy stage. Everything depends on how far you take the boil, and the original leaves that vaguer than a modern cook would like.
- The cold-water test, translated"Boil until it hardens in water" means dropping a little of the syrup into cold water and judging the ball it forms. For a filling that stays spreadable, that is the soft-ball to firm-ball range, roughly 238-245F on a thermometer; taken to the hard-ball stage it sets like candy and will crack rather than spread.
- The instruction that mattersTake it off the heat at temperature and let it cool to about 110F before beating it hard. Beating a hot caramel gives a thin, runny filling; beating a cooled one turns it opaque and thick in a minute or two, and it must go onto the cake immediately after that.
- "Beat all the time"During the boil this means stirring, not whipping — sugar and butter separate and scorch easily. Use a heavy-bottomed pan of at least three quarts, since the mixture climbs sharply when it comes to a full boil.
- CreamNineteenth-century "cream" was unhomogenised and generally richer than today's table cream, though its fat content varied a good deal. Use heavy or whipping cream; half-and-half or milk will not carry the emulsion and the butter is more likely to separate out.
- SugarLight brown sugar gives the expected caramel flavour. Dark brown, with its higher molasses content, makes a noticeably more bitter, almost butterscotch filling and browns faster in the pan.
- ButterThree-quarters of a cup is a stick and a half. Unsalted is the safer choice here; if you use salted, skip any additional salt, though a pinch does sharpen the caramel considerably.
- YieldRoughly two and a half to three cups — enough to fill and thinly top a three-layer eight-inch cake, or to fill two layers generously.
- If it seizesShould the beaten filling stiffen before it is spread, work in a tablespoon of hot cream at a time until it loosens. This is normal with boiled frostings and not a sign that the batch is lost.
Boiled sugar frostings were common in American cake baking from the mid-nineteenth century onward, and were largely displaced in the twentieth by buttercreams made with confectioners' sugar. They exist because the technology was a thermometer-free cook's hands and a cup of cold water — the same cold-water test used for taffy, fudge and pralines, and the reason old candy stages still carry names like "soft ball" and "crack" rather than numbers. Brown sugar made the version here cheaper than one built on refined white loaf sugar, and its molasses content supplies the caramel flavour without the risk of actually caramelising sugar in a dry pan.