Preheat oven to 350 degrees F. Butter a 9-inch round cake pan; dust with flour and tap out excess. Line bottom with parchment. Sift together cake flour and salt three times into a medium bowl.
In a heatproof mixing bowl set over (not in) a pan of simmering water, whisk together eggs and sugar until warm to the touch and sugar has dissolved, about 6 minutes. Remove from heat. With an electric mixer on high speed, beat until thick and pale, about 6 minutes.
Pour milk into a small saucepan; scrape in vanilla seeds and add pod. Heat over medium just until hot; do not let boil. Discard pod. With mixer on low, add milk to egg mixture in a slow, steady stream. Gently fold in flour mixture until combined.
Transfer batter to prepared pan and smooth top with an offset spatula. Bake until cake is golden brown and springs back when touched in the center, about 30 minutes. Transfer to a wire rack to cool 15 minutes. Run a sharp knife around edge and invert cake onto rack, then let cool completely, top side up.
Using a serrated knife, slice cake horizontally into two even layers. Spread bottom layer with pastry cream. Chill until set, about 30 minutes. Remove cake from refrigerator; top with remaining cake layer. Place cake on a wire rack set on a rimmed baking sheet. Pour ganache over cake. Use an offset spatula to swirl top. Transfer to a serving plate and chill until ready to serve.
Chocolate Ganache Icing:
9 ounces bittersweet or semisweet chocolate, finely chopped
1 1/2 cups heavy cream
Place chocolate in a medium heatproof bowl. In a small saucepan, heat cream until bubbles begin to appear around edges; pour over chocolate. Let stand 5 minutes, then stir until smooth. Set aside at room temperature until cool but pourable, stirring occasionally.
Makes about 2 cups
Vanilla Pastry Cream:
6 large egg yolks
3/4 cup sugar
1/4 cup plus 2 tablespoons cornstarch
1/8 teaspoon salt
3 cups milk
2 teaspoon pure vanilla extract
In a large bowl, whisk egg yolks until smooth. Combine sugar, cornstarch, and salt in a medium saucepan over medium heat. Gradually add milk in a slow, steady stream. Cook, stirring constantly, until mixture thickens and begins to bubble, about 5 minutes.
Whisking, slowly pour one-third of the milk mixture into egg yolks. Return mixture to saucepan. Cook over medium heat, stirring, until mixture begins to bubble and thicken, 2 to 4 minutes. Remove from heat; stir in vanilla.
Transfer mixture to a bowl. Cover with plastic wrap, pressing it directly on surface to prevent a skin from forming; refrigerate until chilled, 1 hour or up to 3 days.
Reprinted with permission from Martha's American Food by Martha Stewart, Clarkson Potter copyright (c) 2012