This is not true. Steak n' Shake does not use real ice cream in their milkshakes. They say that their milkshakes are "hand dipped" in their commercials, which is true. However, what they scoop to put in is not ice cream, it's what is called "shake mix" which has the same consistency as ice cream but it's completely flavorless (it's actually quite gross eaten plain). They use that plus syrup of the flavor ordered and that makes the shake.
Anyway, I vote Cook Out, Their shakes are incredible.