A Boston cream pie is a cake that is filled with a custard or cream filling and frosted with chocolate. Although it is called a Boston cream pie, it is in fact a cake, and not a pie.
Owners of the Parker House Hotel in Boston state that the Boston cream pie was first created at the hotel by Armenian-French chef M. Sanzian in 1856. The cake he created, called "Parker House Chocolate Cream Pie", consisted of two layers of French butter sponge cake filled with crème pâtissière and brushed with a rum syrup, its side coated with crème pâtissière overlain with toasted sliced almonds, and the top coated with chocolate fondant.
The commonly known variety is likely derived from the Washington pie, a two-layer yellow cake filled with jam and topped with confectioner's sugar, for which pastry cream of custard eventually replaced the jam, and a chocolate glaze replaced the confectioner's sugar. Today, the cake is topped with a chocolate glaze (such as ganache) and sometimes powdered sugar or a cherry.
The first known attested printed use of the term "Boston cream pie" occurred in the Granite Iron Ware Cook Book, printed in 1878. The earliest known recipe of the modern variant was printed in Miss Parloa's Kitchen Companion in 1887 as "Chocolate Cream Pie".
The Boston cream pie is the official dessert of Massachusetts, declared as such on 12 December 1996.