A mocha is espresso, chocolate and steamed milk — a latte with chocolate in it, essentially. Proportions vary enormously and nobody agrees on a standard.
The name is a coincidence
Mokha is a port on the Red Sea coast of Yemen, and for centuries it was the main export point for the world’s coffee. Coffee from there was described as having a distinct chocolatey character, and the name stuck to that flavour note.
The drink containing actual chocolate arrived far later and has no connection to Yemeni coffee beyond borrowing the word. Yemeni coffee, incidentally, remains among the most distinctive in the world.
Making a good one
Melt real chocolate into the espresso while the shot is hot, rather than stirring cocoa powder into the milk. Dark chocolate at 60 to 70 per cent balances the coffee; milk chocolate makes it very sweet.
Fifteen to twenty-five grams for a double shot is a reasonable starting point.
Then add the milk as you would for a latte, textured and stopped at 60 to 65 °C.
Why it is often disappointing
Two reasons. Syrup instead of chocolate, which is sweet without being chocolatey. And too much milk, which leaves a drink that tastes of neither coffee nor chocolate.
Use a double shot and cut the milk back. The coffee should still be present.
Variations worth knowing
A bicerin, from Turin, layers espresso, drinking chocolate and cream without stirring, served in a small glass — an older and considerably better drink than most mochas.
Questions people actually ask
Is a mocha named after Yemeni coffee?
The name comes from the port of Mokha, historically the main export point for Yemeni coffee, which was described as having a chocolatey character. The drink with actual chocolate in it is a much later and unrelated invention.
Cocoa powder or chocolate?
Real chocolate, melted into the espresso first, gives a considerably better drink than cocoa powder stirred into milk. Powder tends to clump and taste chalky.