1(15.25 ounce) butter or yellow cake mix(I used a butter mix for extra richness)
⅓cupunsalted butter, softened
Preheat oven to 350°F.
In a 10-12 inch cast iron skillet, heat butter and brown sugar over medium heat until butter is just melted. Be careful not to overheat, as mixture will burn.
Remove from heat and place banana slices in an even layer on top of brown sugar mixture. Set aside.
In a large bowl, beat cake mix, water, butter, and eggs with an electric hand mixer or stand mixer with fitted paddle attachment for about 2 minutes, or until thoroughly combined. Pour cake batter over banana mixture.
Bake for about 40-44 minutes, or until a toothpick inserted into center comes out clean.
Remove from heat and run a knife along the edge of pan to loosen cake. Cover the cake with a platter (large enough to cover skillet) and invert. Leave cake inverted for about 5 minutes so that brown sugar mixture seeps slightly into cake.
Remove skillet and let cake cool for about 30 minutes before serving.
I used a 10-inch cast iron skillet and was able to use two medium sliced bananas to cover the brown sugar mixture. If your skillet is larger, you may need to use another banana.