Filling: ½ cup sugar
3 tbsp cornstarch
½ tsp salt
1 large egg plus 3 egg yolks, at room temperature
2 cups whole milk
1 tsp vanilla extract
Cake: Nonstick cooking spray
¼ cup (1/2 stick) salted butter, at room temp
2/3 cup sugar
1 large egg, at room temperature
1 ½ cups cake flour
1 ½ tsp baking powder
¼ tsp salt
¾ cup whole milk
1 tsp vanilla extract
Topping: 1 cup heavy cream
1 ½ cups (12 ounces) semi-sweet chocolate chips
For the filling: In a medium saucepan off the heat, whisk together the sugar, cornstarch, and salt. Stir in the whole egg and the three egg 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, 12-15 minutes. Remove the pan from the heat and stir in the vanilla. Transfer to a bowl 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 350º. 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 ganache topping: 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 the spoon, allowing it to drizzle down the sides. Refrigerate for at least 2 hours before serving.