Chocolate Fudge Frosting

A boiled fudge frosting: essentially fudge candy poured over cake before it sets. The eight-minute boil is the whole recipe, and everything else follows from it.

• The hinge: boil to soft-ball, not by the clockEight minutes is an approximation for a particular pan and stove. What matters is that the syrup reaches soft-ball stage, 234-240F on a thermometer, or forms a ball that flattens when dropped into cold water. Under-boiled it stays runny; past 240F it sets hard and grainy before it leaves the pan.
• Pan and heatUse a heavy saucepan at least three times the volume of the mixture, since cocoa and milk foam up sharply. Stir until the sugar dissolves, then let it boil with minimal stirring to discourage large sugar crystals.
• "Unsweetened powdered cocoa"Plain natural cocoa powder, the same product sold today. Dutch-process cocoa also works and gives a darker, milder result; there is no leavening here for the alkalinity to interfere with.
• Few grains saltA pinch, well under 1/8 teaspoon. With unsalted butter, use a scant 1/8 teaspoon.
• Beating and timingThe recipe says to beat as soon as it comes off the fire; letting the pan stand a few minutes first makes for a smoother result. Beat until the gloss dulls and the frosting thickens to a pourable cream. Have the cake ready and beside you: once it begins to set it stops spreading, and gentle reheating with a spoonful of hot milk is usually the best rescue.
• Vanilla lastAdding it after the boil is deliberate. Vanilla's aromatics drive off at candy temperatures, so it goes in with the beating.
• YieldAbout 3/4 cup, enough to cover the top of an 8- or 9-inch cake or the top of a small loaf. Double it to fill and frost a two-layer cake.