Preheat oven 325°F.

In bowl of stand mixer with paddle attachment, combine flour, baking soda, salt, and sugar. With the mixer running on medium speed, add butter one tablespoon at a time. Continue to mix until crumbly, about 1 minute. Flour must be completely coated in butter.

In separate bowl, combine eggs, banana, and sour cream; mix well. Add about 1/3 of the liquid to the flour mixture and combine on low speed. Once combined, increase speed to medium. Mix until very fluffy, about 3-5 minutes. Scrape down bowl, add remaining liquid, and thoroughly mix, scraping down several times as needed.

Scoop into cupcake liners. Bake 25-30 minutes or until toothpick inserted comes out clean. Cool 10 minutes in pan on rack; remove and cool completely before decorating.