Chocolate Frosting

A three-ingredient cooked frosting that behaves like a small batch of fudge: the sugar syrup does all the work, and the beating decides whether it spreads or crumbles.

• "Health cocoa"Almost certainly plain unsweetened cocoa powder; the term appears to reflect the period's marketing of unadulterated cocoa (and in some books a specific proprietary brand) as against sweetened, starch-extended drinking chocolates. Use natural unsweetened cocoa; Dutch-process works but gives a darker, milder result.
• The hinge is the syrup stage"a thick sirup" is not defined in the recipe, but for a frosting that sets it most likely means somewhere around the soft-ball stage, 234-240F on a candy thermometer. Much under that and the frosting stays loose; much over and it turns grainy and sets before it reaches the cake.
• Beat at the right momentThe recipe has you beat as it cools, until it loses its gloss and thickens. Modern fudge practice would let it drop to about 110F before beating; either way, stop while it is still soft enough to spread, since waiting too long gives a stiff mass.
• Scale and panUse a small heavy saucepan and stir to keep the milk and cocoa from scorching, keeping the sides of the pan clear of sugar crystals. A pinch of salt and a teaspoon of butter off the heat are modern additions, not in the original, but they smooth the texture.
• YieldAbout three quarters of a cup of frosting, enough for the top of one 8-inch layer or a small loaf cake. Double it to fill and cover a two-layer cake.
• If it stiffens too fastBeat in hot milk or cream a teaspoon at a time until it flows again. If it will not set, return it to the pan and cook a few degrees higher.