Hanfstaengle had been very close to Hitler. The headmaster at Hanfstaengle's childhood school was Heinrich Himmler's father. Hanfstaengle was several years older than Heinrich Himmler but they attended the same school and Hanfstaengle remembered Heinrich Himmler as a boy.
Hanfstaengle attended Harvard, became a New York city art dealer, saved the life of Teddy Roosevelt Jr, became a frequent White House guest of Teddy Roosevelt Sr, made friends with Franklin D. Roosevelt, converted the football cheer Harvard, Harvard, Rah! Rah! Rah! into the infamous Sieg Heil, Sieg Heil!
Hanfstaengle also claims that sometime around 1932, Winston Churchill was visiting Munich and that he (Hanfstaengle) got Hitler to come over to Churchill's hotel for a meeting but Hitler got into the Continental Hotel and then decided to not meet Churchill.
Here is some corroboration for Hanfstaengle's claim from a second source.
Winston S. Churchill, The Gathering Storm