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Preheat oven to 350 degrees. Place muffin liners in a 12-cup muffin pan.
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Place the peeled bananas in a large bowl. Using a fork, a potato masher, or a hand mixer, mash the bananas until very few lumps remain.
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Whisk in the egg, sugar, butter, and vanilla.
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In a separate bowl, combine the flour, baking soda, cinnamon, and salt, and stir to combine.
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Add the dry ingredients to the wet ingredients and stir gently until barely combined. Add the strawberries and mix gently until fully combined.
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Evenly distribute the batter between the muffin cups (the cups will be very full).
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Bake for approximately 15-18 minutes or until a toothpick inserted in the center of a muffin comes out clean.