Warm the oven up to 350F. Use a non-stick baking spray to coat a 9x5 loaf pan. Put parchment paper on the bottom and two sides of the pan so it's easy to take the cake out.
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Sift the flour, baking soda, salt, and cinnamon together in a medium-sized bowl. Put away. To make banana mush, use a potato masher or a fork and a small bowl. Put away.
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Melt butter in a microwave-safe basin for 1 minute until completely melted. Wait 3-5 minutes to chill the butter. Mix melted butter and brown sugar in a medium bowl.
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Stir in mashed bananas, sour cream, and vanilla. Mix eggs just till mixed. Avoid overmixing. By now, the batter should be quite moist.
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Whisk dry ingredients in. Make it somewhat thick. In a microwave-safe dish, heat Nutella for 30 seconds.Put ½ of the batter in the loaf pan. It should reach ¼ of the pan height. With a ½ tablespoon, add half of the Nutella to the batter in dollops.
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Stir and drag Nutella with a knife to swirl. Pour remaining batter on top. Repeat Nutella and spread dollops on batter. Swirl Nutella into the batter again with a knife. The pan should barely be ¾ full.
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Bake 60-70 minutes. Bake banana bread till a toothpick comes out clean. Wait 10 minutes in the tin before transferring to a cooling rack. Serve banana bread warm or room temperature.
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