The dessert dates back to the late 1800's and is typically credited to Chef Sanzian of the Parker House Hotel (the present-day Omni Parker House) right in downtown Boston. Other recipes for pudding cakes existed at the time, but this particular version somehow rose above the rest to become a classic.
Why it's called a pie instead of a cake is anybody's guess. One of the more likely theories is that cookware-deprived colonists used shallow "pie" tins for cakes as well as pies and didn't get too fussy about what they called the resulting dishes!
If you can't make it to Boston for the real deal, you can certainly make your own! Take a look at these recipes:
Are you a fan? (P.S. Can you guess what else the Parker House is famous for?!)