Line 2 muffins pans with paper cupcake liners, or spray with cooking spray.
In a medium bowl. sift together flour, baking soda and salt.
In a small bowl, mash bananas and milk with a fork. (You can omit the milk if you want, it just helps to get a creamier consistency with the mashing). Set aside.
In a stand mixer on medium speed, cream together butter and sugar until light and fluffy; about 2 minutes. Stir in egg and vanilla to combine.
Stir in flour mixture to combine, then stir in bananas, sour cream and optional walnuts.
Divide batter among 24 muffins cups (an ice cream scoop helps to keep them equal).
Bake at 350 degrees for about 25 minutes, or until tops are golden brown and a toothpick inserted in the center of a muffin comes out clean.
Cool on a wire rack.