Heat your oven to 350°F (176°C). Prepare two 8″ x 4.5″x 3″ (20cm x 11.5cm x 7.5cm) loaf pan by generously greasing and flouring the interior. For easier bread removal, line the bottom of the pan with parchment paper, but make sure to butter the parchment as well.

In a large bowl, sift together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.

In a separate medium bowl, use a hand mixer to cream the sour cream (or buttermilk), butter, vegetable oil, brown sugar, sugar, and eggs. Beat for about 1 minute until well combined. Add the mashed banana and milk, continuing to beat for another minute.

Add the flour mixture by large spoonfuls, beating for about 30 seconds between each spoonful until all of the flour is incorporated. Stir in the vanilla and walnuts using a rubber spatula, scraping down the sides of the bowl so that the batter is evenly combined.

Pour the batter into your prepared pan. Bake the banana bread in the heated oven for 45 minutes, until a toothpick comes out clean from the center. Remove the banana bread from the oven, allow to cool for about 5 minutes, and then invert the cake onto a wire cooling rack. Remove any parchment paper from the bottom of the banana bread. Allow the banana bread to cool completely.