I don't know what HD stands for. Couldn't be the name of the street (Calle Guzman el Bueno) just off Calle San Bernardo, so I'm assuming it's the owner's initials. :-)
It certainly doesn't look much from the outside but I'm glad I stepped in for my cappuccino con caramel naturale and my molleta con tomate. I was tipped the molleta here is good and tomate is a typical Spanish desayuno.
I do like the vibe inside. I can imagine the crowd when this breakfast place turns into a pub or bar later in the day. Maybe I should try going back another time.
For more details and photos on my food and other discoveries in Madrid (and elsewhere), do check out my lifeisacelebration site. Buen provecho!