In Boston cream pie, a classic first made at the Parker House hotel, the chilled custard filling is sandwiched between two cake layers.
Heat oven to 350 degrees. Lightly butter and flour a 9-inch-round cake pan. Line with parchment paper; 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, mix the eggs and sugar until well combined. Place the bowl over a pot of gently simmering water; whisk until mixture is warm, about 110 degrees, and the sugar is dissolved, about 6 minutes. Remove from heat; place bowl on mixer. With the whisk attachment, beat the egg mixture on high until thickened and pale, about 6 minutes.
Meanwhile, in a small saucepan, combine the milk and split vanilla bean. Place over medium heat until hot, being careful not to let boil. Remove and discard vanilla bean. With egg mixture beating, pour the hot milk into the egg mixture in a slow, steady stream. Turn off mixer. Transfer to a medium bowl; fold in the flour mixture. Pour into prepared pan.
Bake until cake is golden brown and springs back when gently pressed, about 30 minutes. Transfer to wire rack to cool for 15 minutes. Remove from pan; cool completely.
In a large bowl, whisk the egg yolks; set aside. In a medium saucepan, combine sugar, cornstarch, and salt. Gradually stir in the milk. Cook over medium heat, stirring constantly, until mixture thickens and begins to bubble, about 5 minutes. Remove from heat.
Slowly pour the hot milk mixture into the egg yolks while whisking. Return mixture to saucepan. Cook over medium heat, stirring constantly, until mixture begins to bubble, about 2 minutes. Remove from heat; stir in the vanilla extract.
Transfer filling to a medium bowl. Cover with plastic wrap, pressing wrap against the filling to prevent a skin from forming; chill in refrigerator until firm, at least 1 hour.
Split the cake into two layers; spread the bottom half with chilled filling. Place in refrigerator to set, about 30 minutes. Wrap remaining half of cake with plastic wrap; set aside.
In a medium-size heat-proof bowl, or the top of a double boiler, set over a pot of gently simmering water, combine the chocolate and the heavy cream. Stir occasionally until the chocolate melts, about 12 minutes. Remove from heat; set aside to cool for 10 minutes.
Remove cake from refrigerator; top with reserved layer. Transfer cake to serving plate; pour chocolate glaze over top. Allow to set 20 minutes before serving.