Chocolate Frosting II

A cooked sugar-syrup frosting of the fondant type: the hinge is the syrup temperature, since everything after it depends on getting the sugar to just the right saturation.

• "Squares" of chocolateA square meant a 1-ounce block of unsweetened baking chocolate, so this calls for 4 oz. Modern baking bars are often scored into 1/2-oz pieces, so weigh rather than count. Semisweet or chips will make the frosting far sweeter and softer.
• The thread stage"Will thread when dropped from the tip of a spoon" corresponds to roughly 230-235F on a candy thermometer, just below the soft-ball range. Undercooked, the frosting may never set firm; much over it and it can turn grainy and stiff before you can spread it.
• Do not stirThe instruction is deliberate. Stirring boiling sugar syrup seeds crystals and produces a gritty frosting. Swirl the pan if you must, and wipe the inside walls above the syrup with a wet pastry brush to dissolve stray grains.
• Melting the chocolateUse a bowl over barely simmering water or short bursts in a microwave, and keep the chocolate warm and fluid until the syrup goes in. Chocolate that has cooled and stiffened is more likely to go lumpy when the syrup is added.
• Pour graduallyAdd the syrup in a thin stream while beating, as you would for a meringue. A stand mixer or hand mixer does this far better than a spoon; the beating is what breaks the sugar into the fine crystals that give the frosting its opaque, spreadable body.
• Judging consistencyBeat until the mixture loses its gloss and holds a soft ridge, usually a few minutes as it cools. If it stiffens past spreading, beat in hot water a teaspoon at a time; if it stays loose, keep beating as it cools further.
• Vanilla lastAdding it at the end, off the heat, is correct: vanilla flavor is volatile and largely boils away if put in with the syrup.
• YieldThe recipe gives no yield; roughly 2 cups, about enough to fill and cover a two-layer 8- or 9-inch cake. Spread while still warm and workable, as it sets firm.