Preheat the oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners.
In a small bowl, prepare the streusel topping: with your fingers, mix all the streusel ingredients together until well combined. Set aside.
Banana Nut Muffin Batter
In a second bowl, whisk your dry ingredients together: all purpose flour, whole wheat flour, brown sugar, baking powder, cinnamon and salt.
In a third bowl, combine the wet ingredients: mash the banana with a fork, then add the milk, oil, vanilla and egg
Combine the wet and dry ingredients until mixed,a few lumps are ok. Fold in the chopped walnuts.
Spoon batter into prepared muffin cups until full and then sprinkle with the streusel topping.
Bake on an upper middle rack in the oven for 25-30 minutes, or until a knife inserted into the center of a muffin comes out clean.
Let cool for 10 minutes before removing from the tin.
These muffins can be stored at room temperature in an airtight container for up to three days. For longer storage, you can freeze them for up to 2 months. Defrost in a warm oven (300°F/150°C) or just pop in the microwave.