Preheat oven to 350°F (180°C). Line a loaf pan with parchment paper and grease well.
In a large bowl, cream together the softened butter and sugar until light and fluffy.
In a medium bowl, beat the eggs. Mash bananas until smooth, then stir them into the eggs.
Add the banana-egg mixture into the butter-sugar mixture. Stir until well combined.
In another bowl, sift together flour, baking soda, salt, and cinnamon.
Add dry ingredients to wet mixture. Stir gently until just combined. (Do not overmix.) Add vanilla, if using.
Pour batter into prepared loaf pan. Top with walnuts, lightly spray with cooking spray, and sprinkle with cinnamon sugar.
Bake for 50–55 minutes, or until the top is golden and the center is set. (The toothpick test may not be reliable with such a moist loaf — look for a loaf that is set on top and not jiggly.)
Allow to cool in the pan for 10–15 minutes, then transfer to a cooling rack before slicing.