• Slice the layer cake with a sharp serrated knife in half crosswise so that you have top and bottom round layers. Set aside.

Make the Filling:

• Whisk together the milk and the pudding mix for at least 2 minutes until the mix is completely dissolved.
• Gently stir in the whipped topping. Let the filling stand for at least 5 minutes before filling the cake.

Make the Ganache Topping:

• Place the chocolate in the top of a double boiler or in a heat proof bowl set above a pan of simmering water. Stir until the chocolate is just melted. Remove from heat and set aside. Alternatively, melt the chocolate in a microwave at 50% power in 30 second intervals, stirring each time, until just melted.
• Bring the cream just to a boil in a small saucepan over medium high heat. As soon as it starts to boil, remove it from the heat and pour it over the melted chocolate.
• Stir the chocolate and cream together carefully with a silicone spatula until the mixture is smooth and shiny. If it looks like it's not combining, keep mixing; it will come together eventually. Stir in the corn syrup, if using.
• Set the ganache aside to cool slightly so that is is pourable but not runny, or it will run off the edges of the cake when you pour it on.

Assemble the Cake:

• Place the bottom cake layer on a platter or serving dish.
• Carefully spread the filling in an even layer on top of the cake.
• Gently place the top cake layer on top of the filling.
• After the ganache has thickened slightly, carefully pour it in the center of the cake and use an offset spatula to push the ganache toward the edges but not over the edges.
• Refrigerate the cake to set up the ganache. Once the ganache has set, you can loosely cover the cake with foil or plastic wrap. You may want to tent the foil or use toothpicks in the top of the cake to prevent the foil or plastic wrap from sticking to the topping.
• Keep the cake refrigerated until ready to serve. Slice into wedges with a sharp knife.