15 seconds in the microwave will bring leftover muffins back to a fresh out of the oven state.
Whisk together the above dry ingredients in a small bowl and set aside. Mix together the flour, sugar, oats, nuts and butter with your fingertips until it is all combined and has a coarse crumb. Set aside. Beat together the butter and sugar until fluffy. Add the eggs, banana, buttermilk and nuts. Mix thoroughly. Add the dry ingredients to the wet, and mix together just until blended. Grease a 12 cup muffin tin, or use liners. Divide the batter evenly among the muffin cups. Top with the crumb topping and bake for about 20 minutes until a toothpick comes out clean. Cool on a rack.
My policy is to eat one hot from the oven.
You only live once.