Caramel Frosting II

A boiled butter-and-cream frosting in the fudge family: the timing given is a stand-in for a thermometer, and the whole result turns on the temperature the syrup reaches.

• Use a thermometer, not the clockThirteen minutes at a brisk boil is a rough stand-in for the soft-ball stage, around 234-240F. Modern burners and pan materials vary enough that the same time can land anywhere from a syrup that never sets to one that seizes into candy; cook to about 238F and take the pan off.
• Grated maple sugarThis means hard cake maple sugar shaved on a grater, not maple syrup. Granulated maple sugar is the closest current equivalent, measured loosely; 2/3 cup is roughly 4 oz by weight.
• CreamCream sold before standardisation varied a good deal in richness; for this purpose today's light cream is a reasonable stand-in. Heavy cream works and gives a slightly richer, softer set.
• Pan sizeButter, cream and sugar foam up hard once they hit a rolling boil. Use a saucepan at least three times the volume of the ingredients and stir until the sugar dissolves, then leave it alone.
• Beating and the spreading windowCool the syrup to about 110F before beating, then beat until it loses its gloss and thickens. From that point it stiffens quickly, so have the cake ready and work fast.
• If it sets too hardBeat in a spoonful of hot cream at a time until it moves again. If it stays runny, the syrup did not reach temperature and it can be returned to the pan and cooked further.
• YieldEnough to fill and cover a two-layer 8- or 9-inch cake, or to frost a 9x13 sheet.