Preheat oven to 325ºF. Lightly spray pan with vegetable cooking spray.
For topping, combine 1/4 cup flour, brown sugar, ¼ cup walnuts and 2 tablespoons butter in small bowl; mix until mixture resembles coarse crumbs; set aside.
In small bowl, combine 1 2/3 cups flour, baking powder and salt; set aside.
In large bowl, beat 6 tablespoons butter and sugar with electric mixer until light and fluffy. Add eggs, bananas and vanilla; mix well. Add flour mixture to butter mixture; mix just until blended. (Do not over mix.) Gently stir in ½ cup walnuts. Pour into prepared pan. Sprinkle streusel evenly over batter.
Bake 50-55 minutes or until toothpick inserted in center comes out clean. Cool in pan on cooling rack 15 minutes; remove from pan and cool completely.