Maple Sugar Frosting

This is a classic boiled or seven-minute frosting with maple sugar standing in for white sugar. Everything depends on the syrup stage, so a thermometer is worth more here than any amount of watching the fork.

• The hinge: the "rope" stageThe recipe gives no temperature. A syrup that drizzles from the fork in a thread holding its shape briefly is generally taken to fall around the soft-ball range, roughly 235-240F. Short of that the frosting stays soupy; cooked much beyond it, the frosting can set grainy before you finish beating.
• Maple sugar todayHalf a pound of broken maple sugar is about 225 g, which grated or granulated comes to somewhere between a cup and a cup and a half depending how coarse it is. It is sold now as pressed blocks or granulated maple sugar rather than the hard cake an older kitchen would have broken up. Pure maple sugar, not maple-flavoured brown sugar, is what gives the flavour; granulated dissolves in the three tablespoons of water without trouble.
• Two egg whitesThat is about 60-70 g, or a scant 1/4 cup. Have them whipped to soft, glossy peaks before the syrup is ready, since the syrup will not wait.
• PouringAdd the syrup in a thin, steady stream down the side of the bowl with the mixer running, never in a lump. Poured too fast it cooks the whites into threads and the frosting will never smooth out.
• Beating timeWith a stand mixer this takes five to eight minutes on medium-high, until the bowl is no longer warm to the hand and the frosting holds a peak that just bends at the tip. By hand, expect closer to fifteen.
• Egg whites and heatThe hot syrup warms the whites but does not reliably bring them to a pasteurizing temperature. Use pasteurized whites if serving anyone pregnant, elderly, very young or immune-compromised.
• WeatherBoiled frostings misbehave on humid days, absorbing moisture and refusing to set. Cook the syrup two or three degrees higher in damp weather, and frost the cake the same day.
• YieldEnough to fill and cover a two-layer 8- or 9-inch cake, or to top about a dozen cupcakes. Spread it while soft; it crusts within the hour and cracks if worked afterward.