According to The Word Detective, the phrase dates back to 1839, but there was no one particular hotcake fad that inspired it.
I’ve always imagined it to be less about selling hotcakes from a store or restaurant, and more about a family or ranch cook offering up hotcakes and having everyone immediately come running for them. (As if on a day when scrambled eggs were on the menu, people would be, “O.K., time for breakfast,” but if it was hotcakes breakfast would suddenly be a big deal and everyone would want some.)