The primary ingredient in a traditional Boston cream pie is custard or cream filling. Despite its name, Boston cream pie is actually a cake, not a pie. It consists of two layers of sponge cake filled with a rich, creamy custard or vanilla pastry cream. The cake is then topped with a layer of chocolate ganache. This dessert originated in Boston, Massachusetts, in the 19th century and was popularized in part due to its unique combination of flavors and textures. The delightful mix of the rich filling and chocolate icing makes it a classic favorite for many.