Preheat the oven to 350 F.  Butter a 9-inch springform pan and line its base with parchment paper.

Whisk together the flour, baking powder, and salt until well blended.  Set aside.

Place the milk and butter in a small saucepan and set over low heat, stirring occasionally.  Once the mixture begins to simmer, turn it to very low heat until ready to use.

Crack the room temperature eggs into the bowl of a stand mixer and add the caster sugar.  Whisk on medium low speed for a couple minutes, then turn the speed to high and whisk until the mixture has at least doubled in volume and is quite pale.  When the whisk is lifted, a ribbon of batter should fall from it and hold its shape for a few seconds before sinking into the mixture.

Gently fold in the dry ingredients with a silicone spatula.

Trickle in the hot milk mixture and the vanilla extract with the mixer running on low speed.  Use the whisk attachment and stir until well blended.  Be careful not to overmix; you want to keep as much air in the mixture as possible, but still have an evenly mixed batter.

Gently ease the batter into the pan, making sure the batter is level.

Bake at 350 F for 30 minutes, until it is golden brown, is shrinking away slightly from the sides of the pan, and springs back quickly when you lightly press on the center.  Don't open the oven door while it's baking, or the cake may sink.

Let it cool for 10 minutes in the pan, then take it out and place it right side up on a wire rack to cool completely.

Pour the milk into a heavy-based, 2-quart saucepan and set over medium heat, stirring occasionally, until the milk is just barely simmering.

Whisk the egg yolks and sugar with a handheld electric mixer until the mixture is pale and thick.  Beat in the cornstarch and flour until blended.

Gradually trickle the hot milk into the egg mixture, whisking constantly.  Keep whisking until blended, then pour the mixture back into the saucepan.

Whisk the custard constantly over medium heat until it boils and thickens.  Immediately remove it from the heat and stir in the vanilla.

Pour the custard through a mesh sieve to remove any lumps, then pour it into a bowl.  Press a piece of plastic wrap right onto the surface of the custard to prevent a skin from forming.

Chill in the fridge until ready to use.

Pour the cream into a small saucepan and heat over medium heat until it just begins to simmer.  Remove from the heat and whisk in the chocolate until fully combined.

Pour the ganache into a bowl and chill in the fridge until ready to use, stirring occasionally.  This step allows the ganache to thicken, which will result in a thicker layer of chocolate on the Boston cream pie.

Once the cake has fully cooled, gently peel off the parchment paper from its base.  Put it on a large cutting board and get down to eye level with it.  Using a serrated knife, slice it in half, turning the cake as you cut for an even slice.

Place the bottom half on the serving plate or stand and evenly spread the custard in a thick layer on top.  Top with the other piece.