For the Pastry Cream 

1. Beat the egg yolks, sugar, cornstarch, and salt together in a medium bowl until pale and thick.
2. Pour the milk into a small saucepan over medium heat until it starts to simmer. Pour about half of the milk into the egg yolk mixture while whisking to temper the eggs. Then, pour the tempered eggs back into the saucepan while whisking constantly. Continue to whisk while the mixture cooks over medium heat until it is noticeably thickened and starts to bubble.
3. Remove from heat. Strain through a mesh sieve and stir in the vanilla extract and butter until the butter has melted completely. Pour the pastry cream into a shallow, heat-proof container, and cover with plastic wrap, making sure the plastic wrap touches the top of the cream to prevent a skin from forming on top. Let it cool completely at room temperature before putting it in the refrigerator to chill.

For the Cake

1. Preheat the oven to 350°F. Grease and flour the bottom of two 8-inch round cake pans; two inches deep works best. Set aside.
2. Whisk together the flour, baking powder, and salt in a medium bowl.
3. In a separate large bowl, beat the sugar and eggs with an electric mixer on medium speed for about 5-7 minutes or until pale, thick, and bubbly. Fold in the dry ingredients until just combined. Mix in the oil and vanilla extract.
4. Combine the milk and sliced butter in a microwave-safe bowl, and heat in the microwave for 3 minutes or until the butter is melted and the milk is hot. If you have an instant-read thermometer, you want the temperature to be between 190-195°F. Slowly pour a stream of milk into the batter while stirring. After all of the milk has been added, carefully stir the batter to make sure everything is mixed well without deflating it too much.
5. Pour the batter equally into the two cake pans. A digital kitchen scale will help keep the batter even. Bake both cake layers on the middle rack of the oven for 22-25 minutes or until a toothpick inserted in the middle of each cake comes out clean. Let the cakes rest in the pans for 10 minutes before transferring them to a wire rack to cool completely.

For the Chocolate Ganache

1. Combine the heavy whipping cream, chocolate chips, and corn syrup in a small microwave-safe bowl, and heat in the microwave for 1 minute. Stir until smooth, and set it aside to cool slightly.

Assembly

1. Place one of the cake layers on a large plate or cake stand. Spoon the chilled pastry cream on top and spread it out evenly. Place the second cake layer on top, and press it into the pastry cream so the cake is level.
2. Spoon the slightly cooled ganache over the top of the cake, and carefully spread it out so it drizzles down the sides of the cake. Chill the cake for at least 1 hour to help set the ganache before serving.