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Marshmallow Icing
1/2 cup Karo, Red Label 1 cup granulated sugar Few grains salt 1/2 teaspoon vanilla 3/4 cup halved, fresh marshmallows 1/4 cup water Combine the Karo, sugar, salt and water and boil until the mixture threads, 238 degrees F. Add the marshmallows and flavoring and beat until stiff enough to spread. The cake should be cold.
A boiled-syrup icing of the seven-minute family, thickened with melted marshmallows instead of whipped egg white. It comes together fast and sets faster, so have the cold cake and a spatula ready before the syrup comes off the heat.
- Karo Red LabelKaro was sold under colour-coded labels; the red was the darker, stronger-flavoured grade, blue the pale one. Light corn syrup gives the white icing most people expect here; dark corn syrup gives a faint butterscotch note and a tan colour.
- "Boil until the mixture threads"Thread stage is usually given as somewhere around 223-235F, so the recipe's own 238F is really soft-ball. Trust the number over the name and pull the pan at 238F, or the icing will be too soft to hold a swirl.
- Use a thermometerThe margin between spreadable and grainy-hard is a few degrees. Clip on a candy thermometer, and do not stir once the syrup boils, which encourages crystals.
- The marshmallowsPeriod marshmallows were sold in bulk and were generally softer and larger than modern bagged ones, which is probably why the recipe specifies "halved, fresh." Three-quarters of a cup is roughly 2 oz, about six to eight of today's large marshmallows, quartered rather than halved so they melt evenly in the hot syrup.
- BeatingA stand mixer on medium-high takes five to seven minutes to reach spreading stiffness; by hand with a whisk expect closer to ten. Stop as soon as it holds a peak, since overbeaten syrup icings turn dull and start to set in the bowl.
- The cold cake mattersWarm crumb melts the icing and it slides off. Cool the layers completely, and spread the moment the icing is ready, working quickly with an offset spatula.
- WeatherBoiled sugar icings weep and stay tacky on humid days. If the kitchen is damp, cook the syrup two degrees higher and expect a softer set.
- YieldThe recipe gives no yield; this quantity is on the small side, likely enough to top a single layer or sheet, or to cover a small two-layer cake thinly. Best eaten the day it is made; it crusts on the surface and dries by the second day.
Karo corn syrup was introduced in the first years of the twentieth century by the Corn Products Company, later the Corn Products Refining Company, and much of its early market was built through free recipe booklets that existed to demonstrate that a cook could substitute glucose syrup for part of her sugar in candy, pies and icings. The technical argument was sound: corn syrup interferes with sucrose crystallisation, which is exactly why a boiled icing made with it stays smooth rather than turning sandy. The marshmallows are a second industrial shortcut layered on the first. Marshmallow confections take their name from Althaea officinalis, the marsh mallow plant whose mucilaginous root once thickened them, but by the time this recipe was printed they were made with gelatin, sugar and corn syrup and sold by the pound, so melting a handful into hot syrup was a way of getting the airy body of a seven-minute frosting without beating egg whites over a double boiler.