Topping: In a small bowl, mix together brown sugar, chopped walnuts and butter or margarine until mixture resembles course crumbles; Set aside.
Batter: Preheat the oven to 350ºF. Spray a loaf pan or muffin tin lightly with vegetable pan spray. In a large bowl, cream butter and sugar with an electric mixer until light and fluffy. Add the eggs, bananas and vanilla; mix well. Combine the flour, baking powder and salt and add to the butter mixture. Mix until just blended; do not over-mix. Gently stir in the walnuts. Pour into prepared pan; sprinkle topping evenly over the batter.
Bake 50-60 minutes or until a toothpick inserted in the center comes out clean. Cool in the pan on a cooling rack for 15 minutes then remove from the pan and cool completely.