Preheat oven to 350°F (175°C). Grease a 9x5 inch loaf pan and line with parchment. Cream butter, granulated sugar, and brown sugar on medium speed until light and fluffy, about 3 minutes.

Beat in eggs one at a time, then mix in mashed bananas and vanilla until combined.

Whisk flour, baking soda, salt, and cinnamon in a separate bowl. With mixer on low, add flour mixture in three additions alternating with buttermilk, beginning and ending with flour. Mix just until no dry streaks remain.

Fold in chopped walnuts with a spatula. Scrape batter into prepared pan, smooth the top, and add a few walnut halves if desired. Tap the pan gently on the counter.

Bake for 50 to 60 minutes, until a toothpick inserted in the center comes out with a few moist crumbs. Tent with foil if the top browns too quickly.

Cool in the pan for 10 minutes, then lift out onto a wire rack. Slice when completely cool, using a serrated knife for clean cuts.