Preheat the oven to 176 degrees celsius.
Take a large bowl, mix in the butter with the bananas that have already been mashed. Then add in the sugar, egg and vanilla and mix it well together.
Next, add the baking soda and salt together and combine them well together. Then add the flour and mix till it is incorporated properly.
Now, fold in the chopped walnuts. Do not stir the mixture at this point, this make make the muffins fluffier when baked.
Carefully pour this mixture into the muffin pan that has been lined with cupcake liners.
Bake these cupcakes for 25-30 minutes until they turn golden brown in colour. Check to see if they are cooked by inserting a knife into the center of the muffin and it comes out clean or not.
Once cooked, let them cool on a wired rack.
Serve warm for best taste.