Beat butter and cream cheese at medium speed with an electric mixer until creamy. Gradually add sugar, beating until light and fluffy. Add egg beating just until blended. Combine flour and next 3 ingredients; gradually add to butter mixture, beating at low speed just until blended. Stir in banana, walnuts, and vanilla. Spoon batter into a greased and floured 8- x 4-inch loafpans.

Bake at 350° for 1 hour or until a long wooden pick inserted in center comes out clean and sides pull away from pan, shielding with aluminum foil last 15 minutes to prevent browning, if necessary. Cool bread in pan on wire racks 10 minutes. Remove from pan, and cool 30 minutes on wire racks before slicing.