Espresso and a shot of rum or brandy, served hot in a small glass with a sugared rim. The classic carajillo is hot and roughly equal parts coffee and liquor, unlike the modern Mexican version, which is served cold over ice. One glass, five minutes.
The liquor goes in after the coffee, not before: poured gently over the back of a spoon it floats briefly and gives the drink a layered look before it mixes. Brandy is the Spanish classic and rum the Caribbean one, and both are correct. Keep it balanced rather than strong. This is the hot version, which is the original; the carajillo that became popular in Mexico is served cold over ice with Licor 43, and is a different drink under the same name.