Pastry Cream Filling: In a large bowl, whisk together the egg yolks with the granulated sugar until smooth and a little fluffy.
Add the cornstarch to the bowl and continue whisking until there are no lumps in the mixture. Set aside.
In a medium saucepan, heat the milk and the vanilla beans paste over medium heat, whisking occasionally. As soon as bubbles break on the surface of the milk, remove it from the heat.
Slowly add the hot milk into the bowl with the eggs, whisking constantly. This will temper the eggs and keep the eggs from “scrambling”.
Pour the mixture back into the saucepan and continue cooking over medium heat, whisking constantly while you bring it to a boil. The custard will thicken as it begins to boil. Once boiling, allow to simmer gently for an additional 1-2 minutes, then remove from heat.
Add the butter and whisk until the butter has melted and the custard is combined and smooth.
Transfer to a medium mixing bowl and cover the top of the custard with a piece of plastic wrap. Place the custard in the fridge to chill completely. (Tip: The custard can be made one day in advance to save time.)
Cake: Preheat the oven to 350°F. Spray two 8-inch or 9-inch round cake pans with nonstick cooking spray and place a parchment paper rounds in the bottom of each pan. Spray the parchment with cooking spray.
In a medium bowl use a whisk to sift together flour, baking powder and salt. Set aside.
In the large bowl of a stand mixer, use the whisk attachment to beat together the eggs and sugar on medium-high speed until the mixture is pale yellow and thickened. This will take 3-4 minutes.
Add the flour mixture to the egg mixture, slowly, to the stand mixer bowl and beat on low speed until fully incorporated. Scrape the sides of the bowl, when needed.
Use a glass measuring cup to combine the milk and butter. Heat in the microwave for 1 minute or until the butter is melted and the milk is steaming, not boiling. Stir in the vanilla extract. Gradually beat the hot milk mixture into the bowl on low speed beating just until fully combined.
Divide and pour the batter into the prepared pans. Bake the cake layers for 22-25 minutes or until a toothpick inserted into the centers comes back clean and the cakes are lightly golden brown.
Cool in the pan for 5 minutes then invert and flip the cake layers onto a wire rack to cool completely.
Assemble: Once the cakes and pastry cream have cooled, Place one cake onto a platter, serving plate or cake stand. Whisk the pastry cream. Spread the chilled pastry cream over the bottom half of the cake leaving about 3/4-1 inch rim around the edge.
Top with second layer. Gently press down until the filling reaches the edge of the cake. Place in the fridge and chill for 2 hours.
Melt Chocolate: Place the chopped chocolate and corn syrup in a small bowl. Heat the heavy cream in the microwave for 1 minute or just until steaming. Pour the warm cream over the chocolate. Let the chocolate stand for 5 minutes then whisk until smooth.
Pour the ganache over the top of the chilled cake. Spread to the edge of the cake using the back of a spoon or an offset spatula, allowing it to drip down the sides.
Chill for at least 15 minutes to allow the chocolate to set, then slice and serve.