Preheat the oven to 350 degrees. Line a 12-cup muffin pan with paper cupcake liners.
In a medium bowl, combine the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, add the butter and the sugar. Beat with a hand-held mixer until combined. Add the sour cream and the vanilla extract. Slowly beat in the eggs, 1 at a time. Incorporate the dry mixture and wet mixtures together until thoroughly combined. Add the bananas and walnuts, being careful not to over mix. Using an ice cream scoop, fill each cupcake liner 3/4 way full. Bake in the preheated oven until the tops turn golden-brown and when a toothpick, inserted in the middle of the cupcake, comes out clean, about 20 minutes. Remove the cupcakes from the oven to a wire rack and let cool before frosting.