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Preheat oven to 350°F. Lightly grease 12 muffin cups or line with paper baking cups. Set aside. Mix flour, sugar, baking soda and 1/2 cup of the walnuts in medium bowl. Mash bananas with potato masher in large bowl. Add butter, sour cream, egg and banana flavor; mix until well blended. Add flour mixture; stir just until dry ingredients are moistened.
Spoon batter into prepared muffin cups, filling each cup 2/3 full. Sprinkle with remaining 1/4 cup walnuts.
Bake 20 to 25 minutes or until toothpick inserted in center of muffins comes out clean. Serve warm.