What building is at the very center of the courtyard of the US Pentagon?

"Rumor has it that during the Cold War, the Russians never had any less than two missiles aimed at this hot dog stand," DOD Communications Officer Brett Eaton told the Stars and Stripes in 2010. "They thought this was the Pentagon's most top-secret meeting room, and the entire Pentagon was a large fortress built around this hot dog stand. They thought the officers were going to get their top-secret briefings in a protected area, but really they were just going to get lunch."