Preheat oven to 350º F. Butter the bottom and sides of a 10-inch springform pan. Tightly wrap bottom and outside of pan with foil (this will prevent water from seeping in).
In a small bowl combine crushed graham crackers and ½ cup chopped hazelnuts.
Stir in melted butter until combined. Press mixture evenly into bottom of the prepared springform pan. Set aside.
To make batter
Place 1 cup whole hazelnuts into a food processor. Cover and blend until a paste forms. Add olive oil, cover, and pulse until combined. Set aside.
In a large saucepan over low, heat bittersweet and milk chocolate chips until melted, stirring often. Remove from heat and stir in prepared hazelnut butter and Nutella spread. Set aside.
In a large bowl, beat eggs with electric mixer on medium speed until frothy, 1-2 minutes. Gradually add sugar and beat on medium-high until thick and sugar is dissolved, about 6-8 minutes.
Pour chocolate and Nutella mixture into egg and sugar mixture. Stir gently to combine and set aside.
In a clean bowl, beat whipping cream on medium speed until peak form, about 4-5 minutes. Gently fold whipped cream into chocolate mixture. Pour batter onto crust and spread evenly.
Baking and serving
Place filled springform pan into a shallow roasting pan, place on center rack in oven. Pour warm water into the roasting pan to reach halfway up the outside of the springform pan.
Bake 1 hour and 15 minutes. Carefully remove springform pan from water and cool cake on a wire rack for 1-2 hours. Remove foil, cover and refrigerate for at least 4 hours to set.
Let the cake stand at room temperature for 15-30 minutes before serving for best flavor and texture.
Adding the springform pan into a tray filled with water (halfway up sides) will ensure even baking. Water boils at 212º F and will not heat further. This is why the recipe bakes for so long, it's slow and even cooking. The water also evaporates as it boils, creating steam in the oven and adds moisture to the cake.
If you don't have Nutella, use any chocolate hazelnut spread or make your own.
Store leftovers sealed in the fridge for up to 1 week