A caffè shakerato is a double espresso shaken hard with ice until it is cold and foaming, then strained into a chilled glass with no ice in it.
It is the standard Italian summer coffee, and it is dramatically better than iced coffee made by pouring espresso over cubes.
Why shaking is the whole technique
Two things happen in the shaker.
Rapid chilling. The espresso goes from 60 °C to near freezing in about fifteen seconds, which is fast enough that the ice barely melts. Pouring hot coffee over ice, by contrast, melts a lot of it and the drink is watery before you taste it.
Aeration. Hard shaking whips air into the espresso, and the coffee’s own oils and proteins hold it as a thick, stable crema-like foam. That foam is what makes the drink; it lasts several minutes and it is the reason to bother.
Making one
Pull a double espresso. Stir in sugar to taste while it is hot — sugar will not dissolve once the drink is cold.
Add to a cocktail shaker with four or five ice cubes. Shake hard for fifteen to twenty seconds — you want it genuinely cold and your hands uncomfortable.
Strain into a chilled coupe or martini glass. No ice in the glass.
Variations
A splash of amaretto or Baileys is common in bars. Some places shake with a little cream. Purists use espresso, sugar and nothing else.
Not to be confused with
An espresso martini is a cocktail: espresso, vodka, coffee liqueur, sugar. A shakerato is coffee.
Questions people actually ask
Why shake instead of pouring over ice?
Shaking aerates the espresso into a stable foam and chills it in seconds rather than diluting it slowly. Pouring hot espresso over ice melts the ice and gives you weak, watery coffee.
When do I add the sugar?
Into the hot espresso, before shaking. Sugar dissolves poorly in cold liquid, and adding it afterwards leaves it sitting in the bottom of the glass.