Boiled Chocolate Frosting

This is a variation, not a standalone recipe: it assumes you are working from the book's Boiled Frosting, a sugar syrup beaten into egg whites. The only new instruction is when the chocolate goes in.

• What "squares" meansOne and a half ounces of chocolate is one and a half squares of old-style baking chocolate, which was moulded in one-ounce squares. Modern unsweetened baking chocolate is usually sold in 1 oz squares or a 4 oz bar; weigh out 1.5 oz. This is unsweetened chocolate, not semisweet, and the frosting's sweetness comes entirely from the syrup.
• Add it with the sugar, not afterAdd it at the start, not after -- "when put on to cook" means the shaved chocolate goes into the pan with the syrup ingredients at the outset, so it melts and disperses as the syrup heats, rather than being stirred into the finished frosting.
• The temperature that decides everythingA boiled frosting lives or dies by the syrup stage. Most boiled frostings are cooked to about the soft-ball stage (in modern terms roughly 238-240F), though period recipes vary; follow whatever the book's Boiled Frosting directs, then pour the syrup in a thin stream into stiffly beaten egg whites while beating. Under-cooked syrup never sets; over-cooked syrup turns the frosting crumbly before you can spread it.
• Shave, do not chopShaved chocolate melts in the time the syrup takes to come up to heat. Coarse chunks can still be unmelted when the syrup hits temperature, and you cannot stir a syrup at the soft-ball stage without risking crystallisation.
• HumidityBoiled frostings absorb moisture and go sticky and weepy on a damp day. Make it in dry weather if you can, and frost the cake as soon as the mixture holds its shape.
• YieldThe yield depends entirely on the proportions in the book's base Boiled Frosting, which are not repeated here. Spread it while still warm and workable; it stiffens quickly.