For the Pastry Cream:
2 cups half-and-half
½ cup granulated sugar
Pinch of salt
5 egg yolks
3 tablespoons cornstarch
4 tablespoons cold unsalted butter, cut into 4 pieces
1½ teaspoons vanilla extract
For the Sponge Cake:
½ cup plain cake flour
¼ cup all-purpose flour
1 teaspoon baking powder
¼ teaspoon salt
3 tablespoons whole milk
2 tablespoons unsalted butter
½ teaspoon vanilla extract
5 eggs, at room temperature
¾ cup granulated sugar
For the Glaze:
1 cup heavy cream
¼ cup light corn syrup
8 ounces semisweet chocolate, chopped into small pieces
½ teaspoon vanilla extract
1. To Make the Pastry Cream: Heat the half-and-half, 6 tablespoons of the sugar, and the salt in a medium heavy-bottomed saucepan over medium heat until simmering, stirring occasionally to dissolve the sugar.
2. Meanwhile, whisk the egg yolks in a medium bowl until thoroughly combined. Whisk in the remaining 2 tablespoons sugar and whisk until the sugar has begun to dissolve and the mixture is creamy, about 15 seconds. Whisk in the cornstarch until combined and the mixture is pale yellow and thick, about 30 seconds.
3. When the half-and-half mixture reaches a full simmer, gradually whisk the simmering half-and-half into the yolk mixture to temper. Return the mixture to the saucepan, scraping the bowl with a rubber spatula; return to a simmer over medium heat, whisking constantly, until a few bubbles burst on the surface and the mixture is thickened and glossy, about 30 seconds. Off the heat, whisk in the butter and vanilla. Strain the pastry cream through a fine-mesh sieve set over a medium bowl. Press plastic wrap directly on the surface to prevent a skin from forming and refrigerate until cold and set, at least 3 hours or up to 2 days.
4. To Make the Sponge Cake: Adjust an oven rack to the lower-middle position and heat oven to 350 degrees F. Grease two 8- or 9-inch cake pans and cover the pan bottoms with rounds of parchment paper. Whisk together the flours, baking powder and salt in a medium bowl. Heat the milk and butter in a small saucepan over low heat until the butter melts. Remove from the heat and add the vanilla; cover and keep warm.
5. Separate 3 of the eggs, place the whites in the bowl of a standing mixer, reserving the 3 yolks plus the remaining 2 whole eggs in another bowl. Beat the 3 whites at low speed until foamy. Increase the mixer speed to medium and gradually add 6 tablespoons of the sugar; continue to beat the whites to soft, moist peaks. (Do not overbeat.) Transfer the egg whites to a large bowl and add the whole-egg mixture to the mixer bowl.
6. Beat the whole-egg mixture with the remaining 6 tablespoons sugar. Beat at medium-high speed until the eggs are very thick and a pale yellow color, about 5 minutes. Add the beaten eggs to the whites.
7. Sprinkle the flour mixture over the beaten eggs and whites; fold very gently 12 times with a large rubber spatula. Make a well in one side of the batter and pour the milk mixture into the bowl. Continue folding until the batter shows no trace of flour and the whites and whole eggs are evenly mixed, about 8 additional strokes.
8. Immediately pour the batter into the prepared cake pans; bake until the cake tops are light brown and feel firm and spring back when touched, about 16 minutes for 9-inch cake pans and 20 minutes for 8-inch cake pans.
9. Immediately run a knife around the pan perimeters to loosen the cakes. Turn the cakes out and remove the parchment paper. Turn right-side-up and place on a cooling rack. Cool the cake layers to room temperature before proceeding.
10. For the Glaze: Bring the cream and corn syrup to a full simmer over medium heat in a medium saucepan. Remove from the heat and add the chocolate; cover and let stand for 8 minutes. (If the chocolate has not completely melted, return the saucepan to low heat; stir constantly until melted.) Add the vanilla; stir very gently until the mixture is smooth. Cool until tepid so that a spoonful drizzled back into the pan mounds slightly. (The glaze can be refrigerated to speed up to the cooling process, stirring every few minutes to ensure even cooling.)
11. To Assemble: While the glaze is cooling, place one cake layer on a serving plate (if you don't want any stray chocolate, you can place strips of wax paper under the cake for easy cleanup; I didn't put anything under mine). Carefully spoon the pastry cream onto the cake and spread it evenly up to the edges. Place the second cake layer on top, making sure the layers line up properly.
12. Pour the glaze onto the middle of the top layer and let it flow down the cake sides. Use a metal spatula, if necessary, to completely coat the cake. Use a small needle to puncture any air bubbles. Let the cake sit until the glaze fully sets, about 1 hour. Serve the same day, preferably within a couple of hours.