Yield: 12 medium muffins
Spray a medium muffin pan with non-stick vegetable spray. Melt the butter.
In the large bowl of an electric mixer or by hand, mash the bananas. Add the egg, butter, and buttermilk, and mix for a few seconds. Measure the remaining ingredients, except for the walnuts, into another bowl, and mix briefly to avoid clumps of baking soda or salt. Add the dry ingredients to the bananas, and mix only until all the dry ingredients are moist. Fold in the nuts. Using an ice-cream scoop or ladle, put 12 equal portions of batter in the prepared pan. Bake 25 minutes. Remove from the oven, and let cool for 5 minutes. Remove from the pan, and cool thoroughly on a rack. Freeze any muffins you won't be eating that day.