Martha's Boston Cream Pie

Ingredients for the hot-milk sponge cake:
unsalted butter, for cake pan

1 cup sifted cake flour (not self-rising)
1/4 teaspoon kosher salt
4 large eggs
1 cup granulated white sugar, plus more for pan
1/4 cup plus 2 T milk
1 vanilla bean, split lengthwise and scraped

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Ingredients for the custard-cream filling:
6 large egg yolks
1/4 cup sugar
1/4 cup plus 2 1/2 T cornstarch
1/8 teaspoon kosher salt
3 cups milk
2 teaspoons vanilla extract

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Ingredients for the shiny chocolate glaze:
4 ounces semisweet chocolate, coarsely chopped
1/2 cup heavy cream

Preparing the hot-milk sponge cake:
Preheat the oven to 350 degrees F with a rack in the center. Place a baking sheet on the rack below to catch any spills. Butter one 9 x 2-inch cake pan, and line with parchment paper. Dust the bottoms and sides with granulated sugar, and tap out the excess. Set aside.

Sift together the cake flour and the salt. Sift again, twice; set aside. In the bowl of an electric mixer fitted with the whisk attachment, beat the eggs and sugar until well combined. Place the bowl over a pan of simmering water; whisk until the mixture is warm, about 110 degrees F, and the sugar is dissolves, about 5 minutes. Remove from the heat; return the bowl to the mixer. Beat the egg mixture on high until thickened and pale, about 6 minutes.

Meanwhile, in a small saucepan, combine the milk and vanilla bean and seed scrapings. Place over medium heat until hot, being careful not to let boil. Remove and discard the vanilla bean. With the egg mixture beating, pour the hot milk into the egg mixture in a slow, steady stream. Transfer to a large bowl, and fold in the flour mixture. Pour the batter into the prepared pan.

Bake until the cake is golden brown and springs back when gently pressed with a finger, about 30 minutes. Transfer the pan to a wire rack to cool for 15 minutes. Remove from pan; cool completely. Remove parchment paper.

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Preparing the custard-cream filling:
While the cake is baking, make the filling. Prepare an ice-water bath. In a large bowl, whisk the egg yolks; set aside. In a medium saucepan, combine the sugar, cornstarch and salt. Gradually stir in the milk. Cook over medium heat, stirring constantly with a wooden spoon, until the mixture thickens and begins to bubble, about 5 minutes. (It is essential that the mixture come to a boil.) Remove from the heat.

Whisking constantly, slowly pour the hot milk mixture into the egg yolks. Return the mixture to the saucepan. Cook over medium heat, stirring constantly with the wooden spoon, until the mixture begins to bubble, about 3 minutes. (It is essential that the mixture bubble again.) Remove from the heat; stir in the vanilla.

Transfer the filling to a medium bowl. Place the bowl in the ice-water bath. Stir from time to time until the mixture cools; it thickens as it cools. Once completely cool the filling is very thick; it must be covered with plastic wrap, pressing the wrap against the filling to prevent a skin from forming; the filling may be refrigerated overnight at this stage, or it is ready to use.

Use a long serrated knife to cut the cake layer in half horizontally, creating two layers; spread the bottom half with chilled filling, to within only 1/2 inch of the edge so filling does not ooze out. Transfer bottom layer to the refrigerator to set, about 30 minutes. Wrap remaining half of the cake with plastic wrap; set aside.

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Preparing the shiny chocolate glaze:
In the top of a double boiler or in a medium heat-proof bowl set over a pan of simmering water, combine chocolate and heavy cream. Stir occasionally until the chocolate melts, about 12 minutes. Remove from heat; set aside to cool for 10 minutes.

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To assemble the cake:
Remove the cake from the refrigerator; top with the reserved layer. Transfer the cake to a serving platter; pour the warm chocolate glaze over top. Allow to set 20 minutes before serving.