Preheat oven to 350 degrees F (176 C).
First start by making the cocoa dough crust. Place all ingredients except for the water into a food processor and pulse. Add 1 T of cold water at a time until dough forms together.
Wrap with plastic and set in a fridge for 20 minutes.
Once dough is rested, sprinkle some flour over your work surface and roll out the cocoa dough until 1/4 inch thick.
Lay into pie pan and cut off edges, and style however you’d like. I’ve always been bad at crimping edges so I just trim them off.
Take a fork and dock the dough all over. Place pie pan in freezer for 5 minutes to get the butter cold again.
Bake the crust for 15 minutes. Cool down before starting the next step, we are done with the oven.
For the crunchy Nutella layer, place the cream in a small sauce pot and on medium heat.
In a medium bowl, place the Nutella and chopped chocolate. Once the cream starts to rise, remove from heat and pour over Nutella/chocolate mixture. Slowly incorporate cream into the Nutella until it reaches a smooth saucy consistency.
Fold in the chopped hazelnuts, and pour into pie shell. Let this cool and set up for 1 hour in a refrigerator. (or faster in a freezer)
To make the banana custard layer, start by whipping 1 cup of cream until you reach stiff peaks. Set aside.
Next, place ½ cup cream in a small sauce pan.
In a separate bowl, combine yolk, cornstarch, and sugar. Whisk together until thoroughly combined.
Have the mashed banana ready in a separate medium bowl. Add vanilla to this.
Heat the cream on high, and right before it starts to rise (boil), pull from heat and slowly pour a little hot cream mixture into the yolk mixture. You are slowly raising the temperature of the yolk so it does not curdle. Once all the cream is in the yolk mixture, pour it all back into the pot and heat again on medium. Whisk vigorously and do not leave for a second, this will thicken up in no time. Once thickened, quickly pour over the banana mash and whisk to combine and to cool the custard down.
Fold in the whipped cream and continue to fold until all combined.
Now, you can either place cream in a piping bag and pipe the cream layer on the pie OR just smooth it over with a spatula. Finish with some more chopped hazelnuts if desired!
Chill the pie for 2 hours before cutting into it.