Preheat oven to 400°, grease 12 standard size muffin pan cups or use paper liners. Beat together butter and sugar at medium speed until light and fluffy. Add eggs, 1 at a time, beatng well after each addition. Beat in bananas until smooth. Mix together flour, salt, baking powder, and baking soda. Alternately stir flour mixture and buttermilk into egg mixture until dry ingredients are just moistened. Stir in nuts and vanilla. Do not overmix batter; it should not be completely smooth. Spoon batter into prepared pan, filling two-thirds full. Bake until lightly golden, 15-18 min. Transfer muffin-pan cups to a wire rack to cool slightly. Turn out onto rack. Serve warm.