Note on these ingredients: You probably notice some strange amounts. It's because I converted this recipe from its British source. I included the regular measurements for accuracy's sake. On the sugar for the filling where it says to use the ridiculous amount of 7/16 cup of sugar, this was the best I could think of to express it. They want 3 1/2 oz, and 1 cup is 8 oz. What you should probably do is take a 1/8 measuring cup, pour in three full cups of it, and then fill the 1/8 cupper to about half and dump that in as well--that should give you about 3 1/2 oz. For the other ounce measurements for which I provided no conversion, that is because you can use a fluid measurer for fluid ounces and also baking chocolate comes packaged saying how many bars are equal to once ounce. You should have no trouble with those in the United States.
Instructions:
While it's baking, you can make filling. This is the most difficult part and if you can do this you're doing fine. First, take a saucepan and heat the filling's milk in it. You can use the highest level of heat because you want it to boil. (Just watch it carefully or the froth can make it unexpectedly boil over if you look away for a second. I learned that the hard way.) Remove it from the heat just as it boils, you don't want to burn it. In a heatproof bowl (a metal one, preferably), beat the egg yolks until smooth and then add in the sugar until the mixture is pale and yellow. Finally, beat in the flour. Then, add the hot milk to that mixture, stirring constantly and pouring the milk in a steady stream. When it's all been added, take the mixing bowl and place it over a bowl of boiling water (or just use a double boiler if you prefer; you want to cook this mixture, but not directly as it will burn). You must stir it constantly or it may adhere to the bottom of the bowl and burn. Keep stirring it until it is a thick mixture; you want it to become a spreadable pudding, so just use your judgment as to when you think it is a consistency that you could spread on a cake and not have it run AT ALL, you do not want it even remotely liquid. However, you don't want it to be the consistency of cottage cheese, either--stop short of this and remove it from the heat. You'll want to stir in the butter and brandy or vanilla immediately, before it cools. When it is mixed, allow it to cool.
When your cakes are cool, use a sharp knife to slice the tops of the cakes until they make a flat surface; you want one to be able to sit on the other without tilting or being uneven. Put the bottom layer on a serving platter and spread the filling just on its top (not the sides at all). You may have a little left over; if you want to you can make it as thick as you like, but you don't want it to ooze out the sides too much. Place the other cake layer on top (cut side DOWN so that the sculpted pan edge is on top). Now scrape off any squished-out filling so that it looks nice and neat.
Now for the chocolate glaze--it's not Boston cream pie without the glaze! This is easy; melt the chocolate in a double boiler or with the previous method over hot water in a heatproof bowl, and melt the butter while you're at it. Stir them until they are smooth, then take off the heat. Add in the confectioner's sugar; it is highly recommended that you sift it first. You should get a thick paste; add the vanilla and you still have squishy breadcrumbs. Your goal is to get this mixture to be something spreadable; not runny or liquid, but spreadable. Add a tablespoon of hot water--this should be all you need, but if you need a tad more to make it the consistency you want, just add it a teaspoon at a time. Mix it up until it can be spread, then do so; right on the top layer (again, not on the sides!). Dust it with more confectioner's sugar for decoration. It's done! Make sure that whatever's left over, you refrigerate it; that custard filling will go bad long before the cake will.