Preheat oven to 400 degrees. Grease a 12-cup muffin pan. Sift flour, sugar, baking powder and salt into a medium bowl; make a well in center.
Mix milk, egg, oil, banana and walnuts in a small bowl. Pour mixture into well.
Mix batter just until moistened; do not over mix.
Spoon batter into prepared muffin cups. Bake until a toothpick inserted in center comes out clean or until muffins are golden brown, about 15 minutes.