Chocolate Fudge Filling

A straightforward boiled fudge frosting, given in the terse shorthand of a community-cookbook contribution. The only real skill is reading the sugar syrup correctly and knowing when to stop beating.

• The hinge: soft-ball means 235-240FUse a candy thermometer rather than the cold-water test if you have one. Below about 234F the filling will never set and will run off the cake; above 245F it turns grainy and hard before you can spread it.
• "Sweet milk"Ordinary fresh whole milk, specified to distinguish it from sour or buttermilk, which older recipes called for by name. Whole milk gives the best body here.
• Chocolate or cocoa are not interchangeable by volumeThree tablespoons of cocoa powder is roughly 15 g; three tablespoons of grated or chopped unsweetened chocolate is closer to 30 g and brings its own fat. With cocoa, sift it into the sugar dry before adding the milk or it will lump.
• Cool before beatingThe instruction to beat immediately off the heat is the common cause of grainy fudge. Let the pan stand undisturbed until the bottom is warm rather than hot, about 110F, then beat. It will lose its gloss and thicken quite suddenly.
• Pan size and boil-overUse a heavy saucepan at least three times the volume of the mixture. Milk and sugar foam up hard as they approach the soft-ball stage, and a scorched bottom will taint the whole batch.
• Working timeOnce it thickens you have a minute or two. Have the cake layers assembled and the nuts chopped before you start beating; if it stiffens too far, a teaspoon of hot milk beaten in will loosen it.
• YieldEnough to fill and top a two-layer 8- or 9-inch cake; the finished volume depends on how far the syrup is boiled down. Half a cup or so of chopped walnuts or pecans is a common addition, though the recipe specifies no amount.