The milkshake — ice cream blended with milk until thick and creamy, served in a tall glass with a straw — is the American soda fountain's most enduring product and one of the few American foods that has been exported globally without significant modification. The electric blender (Hamilton Beach, 1911) made the modern milkshake possible; the soda fountain counter (drugstores, diners, and drive-ins) made it a cultural institution. The milkshake should be thick enough that the straw stands upright in the glass and thin enough that it can be drunk through that straw — the specific viscosity is the technique.
Ice cream (2-3 scoops — vanilla, chocolate, or strawberry are the classic three), milk (whole — skim produces a thin, icy shake), and optionally flavoured syrups (chocolate, strawberry, malt powder) blended until smooth and thick. Served in a tall glass, often with the overflow in the metal blender cup alongside. The consistency should be thick — spoonable but drinkable — and the flavour should be intensely ice-cream-forward.
1) Ice cream quality = milkshake quality. Premium ice cream (high butterfat) produces a richer, thicker shake. 2) Less milk = thicker shake. Start with ¼ cup per 2 scoops and adjust. 3) Blend briefly — over-blending incorporates too much air and thins the shake. 4) The malted milkshake (malt powder added — the specific slightly sweet, slightly toasty, slightly bitter flavour of malted barley extract) is the traditional soda fountain version and the superior drink.
The malt: malted milk powder (Carnation, Horlicks, or Ovaltine) added to the milkshake transforms it — the malt flavour adds a complexity that plain milkshakes lack. Ask for "a chocolate malt" at any diner that still makes them and you'll understand. In New England, a "milkshake" is milk and flavouring only (no ice cream); the ice cream version is called a "frappe" (rhymes with "cap"). In Rhode Island, it's a "cabinet." The terminology is tribal.
James Beard — American Cookery