Art takes time

This involves making Latte Art for a total of one hour. You might not be able to unlock this in challenge mode so to play it safe, do it during the story. First combine a Coffee - Milk - Milk to make a latte, then choose to make art. Either leave it idle or make some art for one hour. Make sure to watch how long you've been doing this for or set a timer, it will only pop once you serve the drink after!