Preheat your oven to 350°F (175°C) and prepare a muffin tin with liners or grease it.

In a large bowl, whisk together the all-purpose flour, whole wheat flour, baking soda, salt, nutmeg, and cinnamon. Set aside.

In another bowl, mash the ripe bananas and mix in the melted butter, granulated sugar, dark brown sugar, milk, egg, and vanilla extract until blended.

Gently fold the wet ingredients into the dry mixture until just combined; do not overmix.

Fold in half of the chopped walnuts and reserve some for topping if desired.

Evenly distribute the batter among the muffin cups, filling each about two-thirds full.

Bake for 22-26 minutes until golden brown and a toothpick inserted into the center comes out clean.

Remove from the oven and let the muffins cool in the pan for 5 minutes then transfer to a wire rack.