Naturally, the Hoot Hoot I Scream sold ice cream. According to Future Studio, the Hoot Hoot was a real recycling project:
The head rotated; the eyes, made from Buick headlamps, blinked; the sign: Hoot hoot, I scream, used elements of a theater marquee. For over 50 years, Tillie Hattrup ran this L.A.-area refreshment spot designed and built by her husband, Roy in 1926-27. It was demolished in 1979.
More info on the building, with additional digital recreation images, can be found here.
I found this photo via Old Chum when I found these other classic roadside attraction food stand photos. Old Chum (aka Walter Manning of the Old Faithful Shop) says they are from California Crazy: Roadside Vervancular Architecture, compiled by Jim Heimann and Rip Georges; more pics here at his other blog.