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Iced Coffee
Prepare a quart of coffee as for Black Coffee, and have also a quart of well heated milk, but not boiled, and pour the coffee and milk into an ice-cream freezer, sweeten with a little powdered sugar, cover the freezer and place it in a tub of ice and rock-salt, a little higher than the pot of coffee, then turn the handle of the cover in various directions for five minutes, and serve in coffee glasses with powdered sugar separately.
This is a chilled and aerated coffee rather than a frozen one — the five minutes of cranking is meant to cool and froth the mixture, not to set it. Everything hinges on stopping well before anything freezes.
- The hinge: five minutes onlyIn a salted ice bath a quart of liquid will begin to form ice crystals at the canister wall quickly. The aim is a cold, foamy drink poured out at once; leave it longer and you get coffee sherbet.
- QuantitiesA quart is 4 cups. Coffee and milk together make 2 quarts, or roughly eight 8-ounce glasses. "Black Coffee" here is presumably ordinary strong brewed coffee rather than espresso.
- Milk heated but not boiledHold it at about 160-170F. Boiling forms a skin and gives the scalded, slightly sulfurous flavor that shows up sharply once the drink is cold.
- If you have no ice-cream freezerA hand-crank canister set in ice and rock salt is doing two jobs: rapid chilling and whipping air in. A cocktail shaker half-filled with ice, shaken hard in two batches, reproduces both; a blender pulse or an immersion blender gives the froth without the dilution.
- Ice and rock saltCoarse rock salt lowers the brine below 32F so it pulls heat out fast — roughly one part salt to six or eight parts crushed ice by volume.
- Powdered sugarModern confectioners' sugar carries cornstarch, which can leave a faint cloudiness. Superfine (caster) sugar dissolves just as readily without it; a simple syrup is easier still in a cold drink.
- Serving"Coffee glasses" here are probably small stemmed or handled glasses rather than tumblers full of ice cubes. Pour immediately, while the foam holds, and pass extra sugar at the table as directed.
Oscar Tschirky's book dates from the era when the Waldorf's kitchens could count on a steady supply of commercial block ice, and the hand-cranked ice-cream freezer was common enough to turn up here as a general-purpose chilling and frothing machine rather than strictly a dessert tool. Iced coffee itself was known in Europe well before this, in forms such as the French café frappé and mazagran. The half-and-half proportion of coffee to hot milk suggests a chilled café au lait.