This isn't really a pie, as the name suggests, but a two-layered sponge cake. It differs from the original Boston Cream Pie in that we use bananas in the batter to make it even richer. Here's a quick cleanup tip: When assembling the cake, place a baking sheet under the wire rack to catch the drips as you pour on the chocolate. Drizzling on the chocolate can get very messy!
In the bowl of an electric mixer, cream the remaining 10 tablespoons butter and the sugar. On medium speed, add the dry ingredients in 3 additions, alternating with the wet, scraping the sides of the bowl and beating well after each addition.
Pour the batter into the prepared pans. Bake the cake is lightly browned and bounces back when touched with your fingers, about 40 minutes. Let cool for 10 minutes before turning out onto a wire rack to cool completely. Peel off the parchment paper.
To assemble the cake, using a long serrated knife, trim off the rounded top of each cake layer to make them flat. Place the bottom layer on a wire rack set over a baking sheet. Spread the pastry cream evenly over the top. Place the next cake layer upside down on the first. Wrap the cake around the sides with double layers of plastic wrap to keep the pastry cream from oozing out. Gently press down on the top layer. Remove the plastic wrap and scrap off any excess pastry cream.
Slowly pour 1 to 1 1/2 cups of the ganache onto the top of the cake, letting it run down the sides. Let sit for 5 minutes, then, in several applications, slowly pour on the remaining ganache until the cake is completely covered. Chill until ready to serve.
Combine the remaining 2 tablespoons heavy cream, the remaining 1 tablespoon of sugar, the egg yolks, and cornstarch in a medium bowl and whisk until pale yellow. Slowly add 1/2 cup of the hot cream to the egg yolk mixture, whisking constantly until smooth. Add the egg yolk mixture back to the hot cream, and whisk well to combine. Return to medium heat, and cook, stirring constantly with a heavy wooden spoon, until the mixture thickens enough to coat the back of a spoon, about 5 minutes.
Remove from the heat and strain through a fine mesh strainer into a clean container. Cover with plastic wrap, pressing it directly against the surface to prevent a skin from forming. Refrigerate until well chilled, about 4 hours. Whisk until smooth before using.