1. Preheat oven to 350°F. Grease 9-inch tube pan.
2. In large bowl, with mixer at low speed, beat butter or margarine with 1 cup sugar just until blended. Increase speed to high; beat until light and fluffy. Reduce speed to low; add 2 cups flour; beat until well mixed. Spoon 1/2 cup flour-butter mixture into small bowl; set aside.
3. To mixture remaining in large bowl, add mashed bananas, next 5 ingredients, remaining 1 1/2 cups flour, and remaining 1/2 cup sugar. Beat at low speed until well mixed, constantly scraping bowl with rubber spatula. Increase speed to high; beat 2 minutes, occasionally scraping bowl. Stir in chopped walnuts.
4. Pour batter into pan. Sprinkle reserved flour-butter mixture over batter. Bake 55 to 60 minutes, until toothpick inserted in center of cake comes out clean. Cool cake in pan on wire rack 10 minutes; remove cake from pan; cool completely on rack.