A Boston cream pie is a round cake that is split and 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. Created by French chef M. Sanzian at Boston's Parker House Hotel, this pudding and cake combination comprises two layers of sponge cake filled with vanilla custard. The cake is topped with a chocolate glaze. The texture of a Boston creme pie is just like that of a cake filled with a pudding-like creme.
Preheat oven to 350 degrees. Separate three of the eggs. Beat the yolks until they are light and lemon-colored. In a separate bowl, beat the egg whites until they are foamy. Add the cream of tartar and continue beating until stiff but not dry. Beat in one-half cup of the sugar. Fold the yolks into the whites; then fold in one-half cup of the flour and half the salt. Butter and lightly flour the bottom of a 9-inch cake pan. Pour the batter into the pan and bake 35 to 40 minutes. Let cool.
Meanwhile, heat the milk in a saucepan. Beat the remaining egg lightly with the vanilla and remaining sugar, flour and salt. Stir into milk and cook, stirring, until thickened. Cool and chill. Split the cake in half and fill with custard. Top with a creamy chocolate frosting. Chill.
Chocolate butter frosting:
1 pound powdered sugar
1/2 cup butter, creamed
1/8 teapoon salt
1 teaspoon vanilla
2 squares unsweetened chocolate
1/4 cup milk or half and half
Add part of the sugar to creamed butter. Add egg, salt and vanilla. Melt chocolate and add to first mixture. Add more sugar alternately with milk until desired consistency.