Maple Sugar Frosting

This is an Italian meringue in all but name: hot maple syrup whipped into raw whites. The syrup stage is the whole recipe, so work with a thermometer rather than the spoon test.

• Soft maple sugar"Soft" maple sugar is generally taken to mean the softer, moister maple cake sugar rather than hard pressed candy, though the direction to "break sugar in small pieces" suggests it still came in cakes. Granulated maple sugar is the nearest modern substitute; pure maple syrup carries more water than this method allows.
• The water measureThe liquid should be read as 1/2 cup boiling water. That is enough to dissolve a pound of sugar without a long boil-down.
• Thread stage, in numbers"Will thread when dropped from the tip of a spoon" corresponds to roughly 230-235F on a candy thermometer, just short of the soft-ball range. Below that the frosting is likely to stay soupy; much above it and the meringue can set stiff and grainy before you can spread it.
• Do not stir after it boilsThe instruction to boil without stirring is not fussiness. Agitation and stray crystals on the pan wall seed graininess; wipe the sides down with a wet brush instead.
• Pouring the syrupBeat the whites to soft peaks first, then add the syrup in a thin steady stream down the side of the bowl while beating. Syrup poured onto the beaters flings itself into hard threads on the sides of the bowl.
• Egg safetyThe hot syrup warms the whites but does not reliably bring them to a pasteurizing temperature throughout. Use pasteurized whites if the frosting is for anyone very young, elderly, pregnant or immune-compromised.
• Working timeContinue beating until the bowl is no more than warm and the meringue holds a peak. Spread promptly; the surface crusts as it cools and tears if worked cold.
• YieldRoughly enough to fill and frost a two-layer 8- or 9-inch cake.