How to Make It
Make the pudding In a medium saucepan, combine the whole milk and heavy cream with the vanilla bean (not the seeds) and bring to a simmer over moderately low heat, about 2 minutes. Remove from the heat, cover and let the milk steep for 30 minutes.
In a large bowl, whisk the egg whites with the sugar, cornstarch, salt and the vanilla seeds until smooth.
Return the milk mixture to a simmer over moderately low heat; discard the vanilla bean. Ladle 1/2 cup of the hot milk into the egg white mixture, whisking to combine. Continue adding the milk, 1/2 cup at a time, until fully incorporated. Scrape the mixture back into the saucepan, add the butter and cook over moderate heat, whisking constantly but not vigorously, until the custard begins to thicken and bubble, about 3 minutes. Continue to cook for 1 minute longer, whisking, to ensure that the custard gels without a starchy mouthfeel. Strain the pudding through a fine sieve set over a medium bowl. Stir in the vanilla extract and almond extract (if using) and let stand for 5 minutes. Press a sheet of plastic wrap directly onto the surface of the pudding and refrigerate until cold, about 3 hours.
Meanwhile, make the cake Preheat the oven to 350°. Spray two 8-inch round cake pans and line with parchment paper. Spray the paper.
Sift the cake flour into a medium bowl and whisk in the baking powder. In a stand mixer fitted with the whisk, beat the egg yolks, sugar, vanilla, salt and nutmeg at medium speed until thick and doubled in volume, about 6 minutes. Reduce the speed to medium-low and beat in the milk and melted butter, then gently whisk in the dry ingredients until combined.
Scrape the batter into the prepared pans and bake for about 30 minutes, until a toothpick inserted in the center of each cake comes out clean. Transfer to a rack to cool for 10 minutes, then invert onto the rack and leave the pans on top of the cakes to trap the steam. Let cool to room temperature, about 1 hour. Remove the pans and discard the paper.
Fold a 26-inch-long sheet of foil in thirds lengthwise to create a 4-inch-wide band. Using a serrated knife, trim the domed crust from the tops of the cakes. Set 1 cake layer cut side up on a platter. Wrap the foil around the cake to form a collar and secure it with tape. Stir the chilled pudding until smooth, then spread it on top of the cake, keeping the foil collar secure. Top with the second cake layer. Cover the cake with plastic wrap and refrigerate until cold, at least 2 hours or up to 12 hours.
In a small saucepan, bring the heavy cream just to a simmer over moderate heat, about 2 minutes. Remove from the heat and whisk in the dark chocolate until smooth. Scrape the ganache into a glass measuring cup and refrigerate until thickened but still warm, about 15 minutes.
Remove the plastic wrap and the foil collar from the cake. Stir the ganache until smooth and pour it directly onto the center of the cake. Using the back of a spoon, spread the ganache toward the edge to let it drip down in a few places over the side. Serve.