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Preheat the oven to 375 degrees and take an 8-inch square pan and line it with parchment paper.
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In a large bowl, add the mashed bananas, brown sugar, vanilla, egg, oil, buttermilk and cocoa powder. Whisk this mixture well until it’s all very smooth.
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In a separate small bowl, mix the flour, baking soda and salt.
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Gently whisk in the flour mixture into the banana cocoa mixture until just combined into a thick pudding like texture batter.
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Add the chocolate pieces and hazelnuts and fold them into the batter, then pour it into the prepared pan.
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Bake the cake for 35-45 minutes (depending on your oven) until a toothpick inserted in the cake comes out clean (melted chocolate is ok, but not batter!).
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Cool the cake in the pan and then cut it and serve!