Preheat oven at 180C.
In a large bowl, mix butter into the two mashed bananas. Mix in the sugar, beaten egg, and vanilla extract. Add the baking soda and salt over the mixture and mix in. Add the flour last, mix just enough to blend the ingredients, do not overmix.
Add chopped walnuts to the batter and mix them in to distribute evenly through the batter.
Butter a 4×8 inch loaf pan. Line the bottom of the pan with parchment paper, butter the parchment paper too. Pour the batter in the pan. Slice one banana vertically and place them over batter. Bake for 1 hour. Cool on a rack. Remove from pan and slice to serve.
