I'd idly wondered on occasion, over the years, just how the phrase originated, and today I found out:
According to the American Heritage Dictionary of Idioms, a hot cake is essentially a pancake. The term was born in the mid-1800s when the church bake sale was as popular as today’s farmer’s market; people bought the treats quickly, because they were afraid they would get cold. At the time, the world did not yet have cars, penicillin, or Doritos.
Oh - and guess what isn't selling like hotcakes these days? Yep, hotcakes themselves (or more precisely, pancake mix).