Preheat the oven to 350°F / 175°C. Butter and line a 1 lb. loaf pan with parchment paper, to make a hammock for the loaf.
Add the bananas to a large mixing bowl. Mash them well, then add in the oil, sugar, eggs, sour cream, and vanilla. Whisk vigorously until well combined.
Add in the flour, baking powder, baking soda, and salt. Carefully mix the dry ingredients into the wet ingredients just until no flour streaks remain.
Chocolate Loaf
In a small mixing bowl, mix together the cocoa powder and freshly brewed espresso until no lumps remain.
Transfer 340g (1 ½ c.) of the banana batter into the bowl with the cocoa powder. Gently fold the chocolate into the banana batter until the mixture is cohesive.
Alternate spreading a thin layer of the banana batter and chocolate batter into the loaf pan. Once all of the batter has been used, sprinkle the top of the loaf with crushed hazelnuts.
Bake for 50-60 minutes, or until a toothpick/knife inserted in the center of the loaf comes out clean.
Allow the loaf to fully come to room temperature before slicing. Serve and enjoy!