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

Peel ripe bananas and mash coarsely.

Whisk eggs, sugar and vanilla extract in a mixing bowl. Mix in cooled melted butter and cooking oil. Add this mixture to the mashed bananas and mix well.

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

Add wet mixture to flour mixture and stir until ingredients are fully incorporated. Mix in the chopped walnuts.

Line two loaf pans with parchment paper and scoop batter into the pans.

Top with whole and chopped walnuts (optional).

Bake in a preheated oven at 175 C for about 20 minutes on middle rack.

Reduce temperature to 160 C and bake for another 20 to 30 minutes on lower rack.

If you find top of cake turning too brown, cover the top with parchment paper. Test cake with a skewer. It is fully baked when skewer comes out clean.

Remove cake from pan by lifting the parchment paper and leave to cool on rack before cutting.