Make espresso - in a cafetera or however you want (click here for the technique post on Cuban espresso) - but instead of mixing with sugar or milk, leave it black. While the espresso brews, fill espresso glasses (click here for the ones in the picture) halfway with condensed milk - I use a small (2 tsp) cookie scoop to get even amounts. Then once the coffee is ready, put a spoon face down over the cup with condensed milk in it, and pour the espresso slowly over the back of the spoon. It will rest on top of the milk as you serve it, and stay there until you're ready to drink and mix it together.
Suggested Soundtrack: "Devuelveme la Vida" by Bustamante