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Preheat oven to 350 degrees Fahrenheit / 175 Degrees Celsius.
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Whisk together flour + baking soda + baking powder + salt and keep aside.
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In a mixing bowl, mix the butter into the mashed bananas with a wooden spoon. Mix in the sugar + egg and vanilla. Add the flour little at a time and mix until just incorporated. Fold in the chopped walnuts or chocolate chips.
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Scoop mixture into prepared muffin pan lined with cupcake liners till 3/4th full. Divide the streusel topping evenly on all the muffins.
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Bake 20-25 minutes until top is golden brown and once a toothpick inserted into the center of the muffin comes out clean. Remove the muffins and place them on a cooling rack.
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For thick Streusal topping:
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Add oats + flour + brown sugar + cinnamon powder + salt to a bowl and mix well. Add the butter and mix it up with fork.