Preheat oven to 350°F/177°C. Grease a 9-inch oven-safe glass dish. Set aside.
In a bowl, mix together cake mix, oil, water and egg. Pour into prepared pan and bake for 20-25 minutes.
In a small bowl, mix together water and sugar. Stir until sugar is fully dissolved. Heat in the microwave if sugar hasn’t dissolved yet. Mix in liqueur and pour syrup over baked cake. Set aside cake to cool.
For the custard, mix together, sugar, milk, egg yolks, cornstarch, and salt in a sauce pan. Whisk and combine well. Cook in low heat, stirring continuously until the mixture thickens. Once thickened, add in vanilla, salt, butter, and nutella. Refrigerate to cool.
Whip cream until stiff peaks form. Fold in prepared chocolate custard, setting aside about ¼ cup of the custard for the topping.
For the ganache, heat up cream in the microwave for 30-60 seconds until hot but not boiling. Add in chocolate chips, butter, and nutella.
To assemble, pour in half of the custard cream onto the cake layer and level using an offset spatula. Then, make a layer of the chocolate ganache on top of the first custard and cream layer. Top again with another layer of the remaining cream mixture. Top with chopped chocolate of your choice.
Yield: 10-12 servings