Gradually add sifted flour, cornstarch and yeast, and finally add the vanilla.
Butter and flour the 2 moulds, divide the mixture in half between the two moulds, level the surfaces and cook for 25-30 minutes at 170-180°C in a pre-heated static oven.
You can also use 1 single 22cm high-sided mold and cook everything together at 170°C for 40-45 minutes and then cut into 2 discs after cooking.
Once cooked, let it cool.
Add the milk after heating it, then transfer it to the saucepan used for the milk and cook over a low heat, stirring constantly, until you obtain a thick cream.
Place back in the bowl and leave to cool with cling film in contact.
Prepare the syrup by bringing the water with sugar, rum and lemon to the boil.
Place the first disc on the serving plate, wet it with a little syrup and cover it with the cream, then close with the second disc, after having lightly wet this one too.
To make things easier I used an adjustable circle, so as to keep the cake together without getting anything dirty.
Let it rest in the fridge for at least 30 minutes.
Finally proceed with the decoration: coarsely chop (or break up) the chocolate, bring the cream almost to the boil, pour it over the chocolate and stir until it melts completely.
Take the cake back and pour the ganache over it, covering the entire top surface and letting it run a little along the edges, then put it back in the fridge for another 30 minutes.
The Boston cream pie is ready, all you have to do is serve it.