Three overripe bananas, one pan of browned butter, and about an hour — that's all that stands between you and the best loaf you've ever made.
Melt butter in a light-colored saucepan over medium heat. Swirl constantly. It will foam, then go quiet, then turn golden and smell nutty. The moment you see brown specks at the bottom, pull it off the heat and pour into a bowl. Let it cool 10 minutes.
Mash the bananas until nearly smooth — a few lumps are fine, they add texture. Whisk in the cooled brown butter, sugar, eggs, vanilla, and sour cream until combined.
Add flour, baking soda, salt, and cinnamon directly into the wet mixture. Fold gently with a spatula until just combined — stop the moment you can't see dry flour. Overmixing makes tough bread. Fold in chocolate chips and walnuts last.
Pour batter into a greased 9×5 inch loaf pan. Bake at 175°C / 350°F for 55–65 minutes. A skewer inserted in the center should come out with a few moist crumbs — not wet batter, not bone dry. Tent with foil after 40 minutes if the top is browning too fast.
Rest in the pan for 15 minutes, then turn out onto a wire rack. Resist cutting it for at least 30 minutes — the interior is still setting and will be gummy if you slice too early. Worth the wait, every time.