Preheat oven to 350 degrees F.
Using a 9 inch cast iron skillet, melt butter over medium heat until bubbly. Sprinkle brown sugar evenly over melted butter. Do not stir, but allow to sit over heat for just a minute until it starts to dissolve. Remove from heat.
Take the ripest banana and set aside - it will be mashed up and mixed into the cake batter. For the remaining three bananas, peel, cut in half, and then quarter each half lengthwise. Gently place these banana spears in a circular pattern in the butter and brown sugar mixture.
To make the cake batter, combine the flour, sugar, baking powder, and salt in a medium sized bowl. Set aside.
Separately, mix together egg, buttermilk, vanilla, and mashed banana. Add this wet mixture to the dry mixture and stir just to combine. Do not overmix - this will be very similar to pancake batter.
Pour over banana pieces taking care not to change their position.
Bake in preheated oven until golden brown, about 40 minutes. Remove from oven and allow to cool slightly, about 5 minutes, before inverting onto plate.
Store in airtight container. Best if eaten within 1-2 days because bananas will brown over time.