1. 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.
2. Stir together mashed bananas, buttermilk, and vanilla. Stir together flour and next 3 ingredients; 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 overbeat.) Spoon batter into 12 lightly greased muffin cups, filling two-thirds full. Sprinkle evenly with pecans.
3. Bake at 350° for 20 to 25 minutes or until a wooden pick inserted in center comes out clean. Remove from pans immediately, and cool 10 minutes on wire racks.
Tip: If you don't have buttermilk, stir 3/4 tsp. lemon juice or vinegar into 1/4 cup milk.