Preheat the oven to 170 C /350 F and line 9" loaf pan with parchment paper.
In a large bowl, mix with hand mixer brown sugar & butter for 5 minutes, or until creamy and fluffy.
Add eggs and egg whites, vanilla and mix until all is well combined.
Add sour cream, zest from one lemon and mash or blend the bananas. ( I prefer to blend the bananas for nice smooth and more even texture).
In another bowl mix flour with baking powder, salt and nuts. Slowly incorporate the flour mixture into the wet mixture (if you are using chocolate chips, add them now) .
Pour the batter slowly into the parchment lined loaf pan and sprinkle with whole walnuts.
Peel and slice 1 banana lengthwise & place it on the top of the batter. Bake on middle rack 350 F / 170 C for 60 minutes. Then insert toothpick if it's done. If the batter doesn't stick it is done and you can remove it from the oven. If it still sticks, bake for another 10-15 minutes, watching it closely.
If the top starts to brown and the banana bread is not done yet, cover it with parchment paper and continue baking the additional time.
Remove the loaf pan from the oven and let it cool down on wire rack for 15 minutes. Slice the banana bread and serve with ice cream:)