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Preheat oven to 350 degrees F.
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Coat muffin tins with cooking spray.
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Combine mashed bananas, sugar, buttermilk, melted butter, vanilla extract and eggs in the bowl of a mixer. Stir to combine.
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In another large bowl, combine the flours, oat bran, baking powder and soda, salt, and 1 tsp cinnamon.
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Add the dry ingredients to the wet ingredients in two batches and mix on low speed until just combined. Do not over mix or your muffins will be tough.
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Spoon the batter into the prepared muffin tins to 3/4 full.
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In a small bowl, combine the remaining 2 tsp cinnamon and 1 1/2 Tbsp sugar. Sprinkle over the tops of the muffins.
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Bake for around 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean. I recommend rotating the tin halfway through baking for even cooking.