Crust

1. In a stand mixer fitted with the paddle attachment, beat the butter and sugar together just until thoroughly combined.
2. Add the egg yolks one at a time, mixing until incorporated after each addition.
3. Add the flour and the cocoa powder and mix just until combined, scraping the bowl down as needed.
4. Turn the dough out onto a lightly floured surface, and press gently into a ball. Shape into a disc, wrap tightly in plastic wrap and place in the refrigerator for at least an hour.
5. Preheat oven to 350 degrees F.
6. Place the dough on a lightly floured surface and roll out to fit your tart pan, until it is about 1/4 – 3/8 inch thick.
7. Carefully transfer the dough to your tart pan, and press gently to fit it into all the nooks and crannies. If it falls apart at all, it is okay, just piece it together and press it into the pan. Cut off any excess dough, and line the shell with parchment paper.
8. Fill the paper with dry beans or pie weights and bake on the middle rack for 15-20 minutes. When it is completely cooked, remove the pie weights and parchment paper, and let cool completely.

Filling

1. In a medium sized saucepan over high heat, bring the sugar and water up to a boil, and continue to boil until it turns a deep caramel color. Remove from the heat, and quickly and carefully add the cream and butter, and stir to combine. It is normal for it to bubble vigorously.
2. Return the saucepan back to the stove and cook while stirring over low heat until it is smooth. Remove from heat and let cool to room temperature.
3. Once it has cooled, stir in the nutella and the banana until smooth.
4. Pour into prepared crust, and chill for at least 2 hours.

Ganache Topping

1. Place the chocolate in a medium stainless or glass bowl.
2. In a saucepan, bring the cream to a simmer, then remove from heat and pour over chocolate. Let sit for about 30 seconds, then stir with a wooden spoon or spatula until completely smooth.
3. Let the ganache cool for a few minutes, and then pour over filled tart. Smooth quickly with an offset spatula and sprinkle with chopped hazelnuts.
4. Chill until firm.