Melt butter over low heat and set aside to cool down.

Peel ripe bananas and mash coarsely.

Add vanilla extract and sugar to a bowl of eggs and beat well.

Pour the cooled melted butter to the egg mixture and mix well.

Add the egg butter mixture to the mashed bananas and mix gently.

Mix flour, baking powder, baking soda and salt in a large mixing bowl.

Add wet mixture to flour mixture and stir until ingredients are incorporated.

Mix in the chopped walnuts.

Prepare a baking tray with muffin cups and scoop batter about ⅔ full into each cup.

Top with sliced bananas and chopped walnuts (optional).

Bake in a preheated oven at 175 C until golden brown (about 20 minutes).

Test muffin with a skewer. It is fully baked when skewer comes out clean.