Heat oven to 350°F. Grease a 9×5-inch loaf pan and line with a parchment sling.
Crush wafers into fine crumbs. Whisk crumbs with flour, baking powder, baking soda, salt, and cinnamon.
Cream softened butter with granulated and brown sugars until light and fluffy, about 2–3 minutes.
Beat in the beaten eggs gradually, then mix in vanilla until smooth.
Stir in mashed bananas and sour cream until evenly combined.
Add dry mixture in two additions, alternating with the milk. Mix just until no dry spots remain.
Fold in walnuts, if using. Spread batter into the prepared pan and smooth the top.
Sprinkle the top with extra crushed wafers, if desired. Bake 50–60 minutes, until a tester comes out clean.
Cool in the pan 15 minutes. Lift out and cool completely on a rack before slicing.