Preheat oven to 350 degrees, grease and flour a 9 x 5 x 3 inch loaf pan. Cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Sift all purpose flour, soda and salt together, stir in whole wheat flour and add to creamed mixture, mixing well.
Fold in bananas, vanilla and nuts. Pour mixture into pan, bake 50 to 60 minutes or until cake tester comes out clean. Cool in pan 10 minutes, then on rack.