Preheat oven to 350°F. Grease a loaf pan.

Mash bananas in a bowl. Add melted butter and mix well.

Stir in sugar, eggs, and vanilla extract until combined.

In another bowl, whisk together all-purpose flour, whole wheat flour, baking soda, and salt.

Add dry ingredients to wet mixture. Stir until just combined.

Fold in walnuts if using.

Pour batter into loaf pan. Smooth the top.

Bake for 50-60 minutes, until a toothpick comes out clean.

Cool in pan for 10 minutes, then transfer to a rack.