Prepare oven and pan: Preheat oven to 325ºF. Grease and flour a 9- x 5-inch loaf pan.
Make batter: Beat butter and sugar at medium speed with an electric mixer until light and fluffy. Add eggs, 1 at a time, beating just until blended after each addition. Combine flour, soda, and salt. Add flour mixture to batter, and mix just to combine. Add bananas, mixing well.
Add batter to loaf pan: Spoon batter into prepared pan.
Bake banana bread: Bake at 325ºF for 1 hour and 15 minutes or until a wooden pick inserted in center comes out clean and sides pull away from pan, shielding with aluminum foil during last 15 minutes to prevent excessive browning, if necessary. Cool in pan on a wire rack 10 minutes. Remove from pan, and cool 30 minutes on wire rack before slicing. Serve with butter.
