The submarino is a delightful beverage I discovered during my trip to Buenos Aires. I understand it is a traditional hot drink and have seen it described as a simpler way of making hot chocolate. I had this one at one of the Havanna chain of coffee shops. The chain seems to be as ubiquitous in B.A. as Starbucks is here.
To make your own submarino, prepare a tall (12-16 ounce) glass of hot, frothy steamed milk. Break into sections a very good quality 4 to 6 ounce dark or bittersweet chocolate bar and submerge in the hot milk. Serve with a long spoon to stir it and optional sweetener of choice. Add sweetener if using (I didn't), and stir to dissolve the chocolate bar pieces.
Sip enjoying every moment and nibble on the complementary cookie Havanna serves with its drinks or perhaps once of the chain's excellent chocolates or alfajores cookies. (I'll post about alfajores at a later date.)
Buenos Aires Tourist Tip: Most establishments serve some sort of a pastry or chocolate nibble with their hot drinks, so you may want to wait to order that cookie until you see what comes with your cafe con leche.