Sieve the plain flour and the baking powder. Keep aside.
Cream the sugar and butter with a spatula till the mixture is smooth and creamy.
Add the mashed bananas, vanilla essence and walnuts and mix well.
Add the flour mixture and oats and mix well.
Spoon equal quantity of the mixture into each 6 individual paper cups.
Bake in a pre-heated oven at 180°c (360°f) for 15 minutes or until a knife inserted into the muffins comes out clean.
Cool on a wire rack and unmould each muffin.
Serve warm.