Although the name suggests otherwise, Boston cream pie is not a pie, but a cake consisting of two layers of sponge cake which are filled with a rich vanilla custard, while the whole thing is finished with a chocolate glaze, or in some cases, with sprinkled confectioners' sugar.
It was named a pie because the first versions were baked in pie tins, which were more common than cake pans in the mid-19th century. The inventor of the Boston cream pie is a French chef named Sanzian, who worked in the Parker House, a hotel that claims to have served the dessert since its opening in 1856.