Preheat oven to 375°F. Line muffin pan with baking cups; set aside. For streusel topping, in small bowl, combine walnuts and brown sugar; set aside. For muffins, in large bowl, combine all-purpose flour, whole wheat flour, brown sugar, baking powder, cinnamon and salt. In separate bowl, beat milk, oil, and eggs. Add to flour mixture all at once; stir just until moistened. Do not over mix. Gently stir in bananas and nuts. Evenly distribute batter into lined baking cups. Evenly sprinkle streusel topping over each muffin. Bake 18-20 minutes or until golden brown and toothpick inserted in center comes out clean. Cool muffins in pan 5-8 minutes. Carefully remove muffins from pan to cooling rack. Serve warm or cool completely.