Preheat oven to 350 degrees (F). Line a 12-cup cupcake tin with wrappers, or grease.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon together. Set aside.
Mix together the mashed banana, vegetable oil, granulated sugar, brown sugar, milk, egg, and vanilla until well combined.
Pour the wet ingredients into the flour mixture and stir just until moistened.
Gently stir in 3/4 cup walnuts.
Mix the walnuts, brown sugar, flour, and cinnamon together.
Stir in the melted butter then divide the mixture between the muffins, sprinkling on top.
Bake for 18 - 20 minutes or until a wooden skewer inserted into the center comes out mostly clean. A few moist crumbs clinging to the skewer is fine.
Allow the muffins to rest in the pan for 3 to 5 minutes, then remove and cool on a wire rack.
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