Preheat oven to 350°In a large bowl, combine flour, baking soda and salt. Set aside.
Beat butter until smooth and creamy. Add sugar and beat for another minute, at medium speed, scraping down the bowl as necessary. Add vanilla and egg. Mix in bananas (the mixture might curdle, that's oAdd dry ingredients and sour cream.
Grease 6 cup Bundt pan and add the batter to the pan. Tap the pan on the counter a couple times (to remove air bubbles), then smooth if the top. Bake for 55-65 minutes, until a toothpick inserted into the center comes out clean. Place the pan on a wire rack until it is cool enough to handle.
Then carefully unmold the cake and place it back on the rack to cool completely.