Preheat oven to 350F.
In a mixing bowl, combine flour, baking soda, and salt. Stir using a wire whisk until well blended. Set aside.
Combine softened butter, vanilla extract, and sour cream in another bowl. Start to cream the mixture; use a blended, if possible for better results. The beater blade of your stand mixer needs to be used in this process.
Add the mashed bananas and walnuts while beating and then gradually add the flour mixture. Continue to beat until the texture becomes smooth.
Scoop the mixture and arrange in small rectangular cups or cupcake cups. Put the rectangular cups over a baking tray. If using cupcake cups, use a muffin pan.
Bake in the oven for 20 to 30 minutes or until it turns golden brown
Remove from oven. Serve. You can also top with whip cream so that kids will like it more.