Preheat oven to 350°F. Line muffin tin wells with liners.
Cream the butter, white sugar and brown sugar together in a large mixing bowl. Add the egg and vanilla and blend again and then add the mashed bananas.
Sift together the flour, nutmeg, cinnamon and salt. Slowly add to the banana batter.
Fold in the walnuts by hand. Do not overmix the batter.
Fill each muffin tin approximately 1/2-3/4 full. Bake for 15-20 minutes or until tops spring back when gently pressed.
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