Boston Cream pie is not really technically a pie although it is called a pie.
In reality Boston Cream pie is actually a cake that is made with 2 layers of cake and is filled with a vanilla custard and then topped with a chocolate frosting.
The dessert Boston Cream pie got it's name from the type of pan that it was originally baked in during the mid 1800s.
Boston Cream pies are basically a cake with a cream filling and not actually a pie.
While it's not a pie, the Boston Cream pie is called a pie because the Boston Cream Pie was baked originally in pie pans during a time when cake pans and pie pans were used interchangeably.
In the 1850s when the Chef at Boston's Parker House Hotel developed the Boston Cream Pie, the term pie pan was used to refer to the baking container, even when they were baking a cake.
So as a result the Boston Cream Pie which is technically a cake became known as a pie and is a name that stuck even despite Boston Cream Pies true nature as being a cake.
The Boston Cream Pie filling is a vanilla pastry cream, which is essentially a custard that is made from milk, eggs, sugar, cornstarch (or flour), and vanilla extract.
The filling of the Boston Cream Pie is then placed between two layers of cake, often a sponge cake, and topped with a chocolate glaze.