To make the streusel: In a medium bowl, whisk together flour, sugar, and cinnamon. Add butter and mix with your fingertips until the mixture lumps into unevenly sized crumbs. Add hazelnuts and mix. Refrigerate until ready to use.
Preheat oven to 325°F/160°C. Grease a 9x5-inch loaf pan.
To make the cake: Place bananas in a large bowl and mash them well using a fork or potato masher. Add oil, sugar, eggs, and vanilla extract, and mix until combined. Add the other ingredients, except for Nutella, to the bowl, and whisk just until combined. Set bowl aside.
Recipe adapted from Bake from Scratch magazine, Recipe by Laura Crandall