Preheat oven to 350 degrees.
In medium size bowl, whisk together flour, baking soda, and salt.
Add sugar and stir until well combined.
Add vanilla, mashed bananas, melted butter (unsalted), beaten egg, and chopped walnuts (if using). Stir until well combined. Set aside.
Thoroughly grease bottom and sides of a loaf pan (8 x 5 x 3) with vegetable oil cooking spray, vegetable oil, or butter.
Pour in batter. Place two banana slice on top. Bake 40 to 45 minutes or until knife inserted in center comes out clean.
Although delicious warm, tastes much better after 24 hours in the fridge.
After baking, cool completely. Place in sealed container or zippered freezer bag. Refrigerate overnight. Slice and serve cold!