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Pre-heat the oven 375 Lightly grease a 12-cup muffin pan with butter.
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For the walnut topping, in a small bowl, combine the melted butter, brown sugar, and 1/2 c chopped walnuts and set aside.
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In another bowl using an electric mixer on med speed, combine the mild, oil, eggs, vanilla, and mashed banana until there are no lumps. Make a well in the flour mixture and add the banana mixture all at once. Using a spatula or large spoon, stir gently until just barely combined. Fold in the 1/2 c chopped walnuts and diced banana. The batter should be a little lumpy, so don't overmix.
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Divide the batter equally into the 12 muffin cups and, using your fingers, sprinkle each muffin with the walnut topping. Bake for 25 min or until lightly browned.
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Immediately remove the muffins from the pan to a wire cooling rack.