6 large egg yolks
1/2 cup granulated sugar
1/3 cup cornstarch, sifted
2 cups whole milk
3 tablespoons unsalted butter, cut into tablespoons
2 teaspoons vanilla extract
preheat the oven to 350 degrees
To make the custard filling:
In a medium bowl, whisk together the yolks, sugar, and cornstarch; set aside.
In a medium saucepan, bring the milk to a gentle boil. Remove the pan from the heat and whisk about 1/2 cup of the hot milk into the yolk mixture.
Return the entire mixture to the saucepan containing the remaining milk. Place over medium-high heat and bring to a boil, whisking constantly. Continue to boil, whisking constantly, for 1 minute, or until thick.
Remove the pan from the heat, scrape the bottom of the pan with a spatula, and whisk until smooth. Whisk in the butter pieces until melted. Whisk in the vanilla. Quickly strain the custard through a fine-mesh sieve into a medium bowl.
Cover with plastic wrap, pressing it directly onto the surface. Let cool to room temperature and then refrigerate for 2 hours, or until well chilled.
To make the sponge cake:
Butter the bottom and sides of two 9-inch round cake pans. Dust the pans with flour and tap out the excess.
In a small bowl, stir together the flour and salt. In the bowl of an electric mixer, whisk together the eggs and sugar.
Set the bowl over a saucepan of simmering water, making sure that the bottom of the bowl does not touch the water. Heat the egg mixture, whisking constantly, until the eggs are warm.
Transfer the bowl to the electric mixer stand and, using the whisk attachment, beat on high speed for about 10 minutes, or until the mixture has tripled in volume. Reduce the speed to low and beat in the lemon zest and vanilla.
Resift one-third of the flour mixture over the batter and gently fold it in with a rubber spatula. In two more additions, sift in the remaining flour mixture, again folding in gently.
Place the melted butter in a small bowl. Scoop about 1/2 cup of the cake batter into the bowl and stir until blended.
Fold this mixture into the remaining cake batter. Scrape the batter into the prepared pans and smooth the tops with the spatula.
Bake the cakes for 28 to 30 minutes, or until the tops spring back when lightly touched. Cool the cakes in the pans on wire racks for 15 minutes. Invert the cakes onto the racks and cool completely.
To assemble the cake:
Using a long, serrated knife, cut each cake layer in half horizontally.
Remove the custard filling from the refrigerator and whisk until smooth.
In an electric mixer set on high speed, beat the heavy cream until it forms soft peaks. Fold one-third of the whipped cream into the custard to lighten it. Fold in the remaining cream.
Place 1 cake layer, cut side up, on a serving plate. Scrape about 1 cup of the custard filling onto the layer and, using a small offset metal spatula, spread it into an even layer.
Repeat with the remaining cake layers and filling, ending with a cake layer.
Refrigerate the cake while making the chocolate glaze.
To make the chocolate glaze:
Place the chocolate and the cream in a medium bowl and set the bowl over a saucepan of simmering water, making sure that the bottom of the bowl does not touch the water.
Heat the mixture, stirring often, until the chocolate is melted and the mixture is smooth.
In a small container, dissolve the espresso powder or instant coffee in the hot water. Stir the coffee mixture and vanilla into the chocolate glaze.
Remove the cake from the refrigerator. Pour the warm glaze over the top of the cake, allowing some of it to drizzle down the sides. Serve the cake immediately, or refrigerate and bring to room temperature before serving