Preheat the oven to 350 F. Line a (12-cup standard) muffin tin 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 granulated sugar. Beat with a hand-held mixer until combined. Add the sour cream and vanilla. Slowly beat in the eggs, one at a time. Incorporate the dry mixture and wet mixture together until thoroughly combined. Add the bananas and 1/2 cup walnuts, being careful not to over mix.
Using an ice cream scoop, fill each cupcake liner three-quarters of the way full. Bake in the oven until the tops turn golden-brown and a toothpick, inserted in the middle of the cupcake, comes out clean, 18 to 20 minutes. Remove the cupcakes from the oven to a wire rack and let cool before frosting.
For the frosting:
In a large bowl, add the cream cheese and butter. Beat together until incorporated. Add the molasses and mix well. Stir in the confectioner's sugar, milk and vanilla.
Heavily frost the cupcakes and garnish with the remaining 1/4 cup walnuts.