Preheat the oven to 350 degrees F. Coat the inside of a 9-inch x 13-inch baking dish with non-stick cooking spray.
Prepare cake according to package directions. Pour into the prepared baking dish and cook according to the package for a 9x13 pan. The cake is done when a toothpick inserted in the center comes out clean. Remove cake from the oven.
While the cake is cooking, prepare the vanilla pudding. Place pudding mix and 2 cups milk into a bowl. Whisk for 2 minutes until the pudding starts to thicken. Place in the fridge.
Next, make the pastry cream. In a medium bowl, whisk together milk and egg yolks. To a medium saucepan add the sugar, cornstarch, and salt. Whisk to combine and place over medium heat. Pour milk mixture into the saucepan while whisking. Cook until the mixture begins to thicken and bubble. Remove from the heat and whisk in vanilla. Transfer pastry cream to a bowl and cover with plastic wrap touching the surface. (This will prevent a skin from forming on top.) Place in the fridge.
Now it's time to make the ganache. Add the chocolate and corn syrup to a mixing bowl. Heat the cream in the microwave for 1.5 minutes until boiling. Pour the hot cream over the chocolate and let that sit for 2 minutes. Then whisk the chocolate until shiny and smooth. (It will look broken and weird, just keep whisking.) Cover with plastic wrap and leave on the counter.
Once the cake is out of the oven, use a fork or the back of a wooden spoon to poke holes all over the top of the warm cake. Spread the vanilla pudding over the cake. Spread the pastry cream over the pudding. Chill for 1 hour.
Add dollops of ganache frosting on top of the pastry cream. Use an offset spatula or the back of a spoon to gently spread the frosting over the top of the cake to the edges of the pan. Chill for 1 hour, up to overnight. Cut into 12 slices and serve. Store in an airtight container in the fridge.
If you prefer, you can heat the heavy cream in a small saucepan until it just begins to boil. Stir often to prevent scalding.
You can double the pastry cream recipe and omit the pudding if you prefer a more traditional Boston Cream pie taste. I like the combination of vanilla pudding and pastry cream. (In contrast, I think ALL vanilla pudding is too sweet and you don’t get the same taste as the classic cake.)
If you can’t find vanilla cake mix, you can use butter cake or yellow cake mix instead.