In a medium bowl, whisk together the yolks, sugar and the cornstarch.

In a medium saucepan bring the milk to a gentle boil, be careful it doesn't boil over.

Remove the pan from the heat and whisk about 1/3 cup of the hot milk into the yolk mixture.

Return the entire mixture to the saucepan.

Over medium-high heat bring the mixture to a boil, whisking constantly for 1 minute. Remove the pan from the heat and whisk until smooth.

Whisk in the butter until melted.

Immediately strain the custard through a fine-mesh sieve into a medium bowl.

Whisk in the vanilla extract.

Cover the surface of the custard with plastic wrap and let cool to room temperature. Refrigerate for two hours, or until well chilled.

Preheat the oven to 350 degrees F.

Grease the bottom and sides of a 9-inch cake pan.

Wisk together the cake flour, baking powder, and salt. Set aside.

In a small saucepan, combine the milk and butter.

Heat over medium heat just until the butter is melted. Remove the pan from the heat and set aside.

In a bowl of an electric mixer using the whisk attachment, beat the eggs at high speed until blended, about 1 minute. Gradually add the sugar and vanilla extract and beat until pale and tripled in volume, about 6 minutes.

Gently fold one-third of the flour mixture in to the egg mixture. Repeat with the remaining flour mixture in two more additions.

Reheat the milk mixture to just under a boil.

Add it all at once to the egg mixture and gently fold it in. Scrape the batter into the prepared pan.

Bake the cake for 20-25 minutes, until it springs back when lightly touched and a cake tester inserted into the center comes out clean.

Cool the cake in the pan on a wire rack for 10 minutes. Run a paring knife around the edge of the pan and invert the cake onto the wire rack.

In a small saucepan, bring the cream to a boil.

Remove from the heat and add the chocolate to the pan.

Stir until the chocolate is completely melted and the glaze is smooth.

Stir in the vanilla extract. Transfer the glaze to a small bowl.

Cover the surface of the glaze with a piece of plastic wrap and let cool for about 10 minutes before using.

Transfer the custard filling to the bowl of an electric mixer. Add 1/4 cup heavy cream and using the whisk attachment, beat the mixture at high speed until it is light and forms soft peaks, about 1 minute.

Using a long serrated knife, cut the cake horizontally in half to make 2 layers. Save the smooth bottom layer for the top of the cake and place the remaining cake layer cut side up on a serving plate.

Scrape the filling onto the layer and spread it into an even layer using a small spatula.

Top with the other cake layer, cut side down. 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.