For the filling: In a medium saucepan off-heat, whisk together the sugar, cornstarch and salt. Stir in the whole egg and the 3 yolks, and then stir in the milk. Turn the heat on to medium high and cook the mixture, uncovered, stirring constantly, until the custard thickens, 12 to 15 minutes. Remove the pan from the heat and stir in the vanilla. Transfer to a bowl, cover with plastic wrap and refrigerate while you make the cake. Be sure to place the plastic wrap directly onto the custard to prevent a skin from forming.
For the cake: Preheat the oven to 350 degrees F. Spray a 9-inch round cake pan with cooking spray and set aside.
Using an electric mixer, cream the butter and sugar together until fluffy, about 5 minutes. Add the egg and mix until completely combined. Sift together the cake flour, baking powder and salt in a bowl. Add the flour mixture to the butter mixture alternating with the milk, beginning and ending with the flour. Add the vanilla.
Pour the batter into the prepared pan and bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean. Remove the cake from the oven and allow to cool in the pan for 10 minutes, then turn out onto a rack to cool completely, about 20 minutes.
For the topping: To make the chocolate ganache, in a medium saucepan over medium-high heat, bring the cream to a boil, then immediately remove from the heat. Add the chocolate chips to the cream and stir until completely melted.
To assemble, slice the cake layer in half horizontally. Spread the custard filling onto one layer, then top with the second layer of cake. Spread the ganache over the top of the cake using an offset spatula or the back of a spoon, allowing it to drizzle down the sides. Refrigerate for at least 2 hours before serving. Store covered in the fridge for up to 2 weeks.
If you don't have cornstarch, substitute all-purpose flour. One tablespoon cornstarch equals 2 tablespoons flour.