Make the cake: Beat the sugar and egg together at medium-high speed using a stand mixer fitted with a paddle attachment (or hand mixer) until very thick, about 3 to 4 minutes (butter will fall from the paddle in thick ribbons). Beat in the vanilla extract.
Using a sifter, combine the flour, salt, and baking powder in a small bowl. Add to the egg mixture and beat on low speed just until combined, about 1 minute.
In a saucepan set over medium heat, bring the milk and butter just to a simmer. Remove the pan from the heat, and stir the mixture until the butter is completely melted.
With the mixer going, slowly add the hot milk mixture to the cake batter, mixing until everything is well combined. Scrape the bowl and mix briefly, just until smooth. The batter will be very thin.
Wrap the cake pans with Bake-Even strips, if desired (you won't be a need to level the cakes later). Divide the batter into the prepared pans, checking carefully for lumps of flour (smash or remove them as you spot them).
Bake the cakes for 30 to 35 minutes, until they're a deep golden brown and beginning to pull away from the sides of the pan.
Remove the cakes from the oven, run a table knife around the edges, and cool in the pan for 10 minutes. Turn them out onto a rack to cool completely.
Make the pastry cream: Bring the milk, vanilla bean (if using), and half of the sugar just to a simmer in a large nonreactive saucepan.
Whisk the egg yolks in a mixing bowl and gradually add the remaining sugar. Whisk in the cornstarch to combine.
Take about a ½ cup of the scalded milk and slowly add it to the yolk mixture, whisking constantly. Return the yolk mixture to the pan and cook, whisking vigorously, until the cream boils and is well thickened. Allow the pastry cream to boil approximately 1 minute longer, stirring constantly. Don't overcook the pastry cream to avoid overcooking the eggs.
Remove the pastry cream from heat. Fold in the vanilla extract (if using) and butter; stir until the butter melts. Do not overmix, as this will thin the custard.
Strain the custard through a fine mesh strainer into a clean mixing bowl, removing the vanilla bean. Chill in an ice bath, then refrigerate until needed, covered with plastic wrap on the surface of the custard (this prevents a buildup of “skin” on the surface of the custard).
Assemble the cake: Level the cakes (if needed). Place one of the cakes, right side up, on a cake plate. Top with a good amount of the pastry cream, spreading it right to the edges (you want at least ½-inch of filling inside when the cake is assembled). Place the top layer, upside down, on the filling.
Prepare the ganache: Place the chopped chocolate in a bowl. Heat the cream to a simmer either on the stove in a small saucepan or in the microwave. Pour the cream over the chopped chocolate, cover, and let sit for 2 minutes, then stir together until smooth and lump free. Add the corn syrup and vanilla and stir well.
Pour the ganache over the filled cake and let set for a few minutes. Serve and enjoy!
Since chocolate ganache is just equal parts by weight of chocolate and heavy cream, a kitchen scale is helpful here.Chop the chocolate with a serrated knife. It cuts it cleaner than a chef's knife.Tempering the eggs (bringing them slowly up to temperature by slowly adding the hot milk mixture) helps to avoid little pieces of cooked egg in your pastry cream. If however you do see some egg pieces, straining the cream through a fine mesh strainer will take care of them.Don’t toss that spent vanilla bean…instead rinse the bean, then place in a jar and cover with vodka to make your own vanilla extract.You can make the cakes a day ahead and chill wrapped in plastic wrap. Bring to room temperature before assembling the cake.Store any leftovers in the fridge, wrapped in plastic.For a full-sized cake, double all the ingredients. Bake the cake in two 9-inch round cake pans for 35 to 40 minutes.