A cortadito is Cuban espresso cut with an equal amount of steamed milk. What separates it from a Spanish cortado is the sugar, and specifically when the sugar goes in.
Espuma
The defining Cuban technique. A spoonful of sugar goes into a cup, the first few drops of the espresso are poured onto it, and the two are beaten vigorously with a spoon until they form a pale, thick, caramel-coloured foam called espuma.
The rest of the shot is then poured through it, and the espuma rises to the top as a sweet crema.
The drinks built on it
Café cubano (cafecito): the espresso with espuma, alone, in a tiny cup.
Cortadito: that, cut with an equal volume of steamed or evaporated milk.
Colada: a large serving of café cubano with several small plastic cups, meant to be shared — a workplace institution in Miami.
The coffee
Traditionally dark-roasted, often with robusta, brewed in a moka pot at home and on an espresso machine in a cafe. It is not subtle and it is not meant to be.
Making one
Whip a teaspoon of sugar with the first drops of a double espresso until pale and thick. Pour the rest through. Add an equal volume of steamed milk.