Heat the oven to 350°F. Grease a 9x5-inch loaf pan and line it with a parchment sling hanging over both long sides.
Mash the bananas in a large bowl until mostly smooth, leaving a few small lumps. You should have about 1½ cups.
Whisk in the melted butter and sugar, then add the eggs one at a time, followed by the sour cream and vanilla. Whisk about 30 seconds until glossy and uniform.
Sprinkle the flour, baking soda, salt, and cinnamon over the surface. Fold with a spatula only until the dry streaks disappear — do not overmix.
Fold in ¾ cup of the walnuts with two or three strokes.
Scrape the batter into the pan, level the top, and scatter the remaining 2 tablespoons of walnuts over it.
Bake 60–70 minutes, until a toothpick inserted in the center comes out with moist crumbs but no wet batter. Tent loosely with foil if the top browns before 40 minutes.
Cool in the pan 15 minutes, then lift out by the parchment and cool on a rack at least 1 hour before slicing.