Preheat oven to 350 degrees. Grease a 10-inch cast iron pan. Sprinkle 1/4 cup sugar on the bottom. Shake out the excess.
In a medium bowl, mix together the softened butter and dark brown sugar. Spread into the bottom of the pan.
Arrange the bananas, cut side up on top of the brown sugar mixture, cutting pieces as needed to fit the empty spaces.
In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
Using an electric mixer, cream the remaining 1 cup sugar with the 1 stick of cold butter until light and fluffy, about 5 minutes.
Beat eggs in one at a time.
Beat in vanilla extract.
With mixer on LOW speed, add 1/3 of flour mixture, followed by 1/2 of milk.
Repeat and then add remaining flour.
Mix in peanut butter, being careful not to overmix.
Pour batter into prepared pan. Set pan on a baking sheet to catch any overflow and bake for 1 hour.
Let cool 10 minutes and then invert onto a serving platter.