Beat butter at medium speed with an electric mixer until creamy. Gradually add brown sugar, beating until light and fluffy. Add eggs, 1 at a time, beating just until blended after each addition.

Stir together mashed bananas, buttermilk, and vanilla. Stir together flour, baking soda, baking powder and salt; add to butter mixture alternately with banana mixture, beginning and ending with flour mixture. Beat at low-speed just until blended after each addition. (Do not over beat.) Spoon batter into 12 muffin cups, filling 2/3 full. Sprinkle evenly with pecans... or not.

Bake at 350 degrees for 20-22 minutes or until wooden toothpick inserted into center comes out clean.