Preheat oven to 350℉ (180℃)
Lightly grease each cast iron skillet with melted butter or baking spray.
Combine flour, baking soda and salt in a bowl. Whisk well to combine. Set aside for later.
Mash the bananas until mostly smooth with minimal lumps.
Combine granulated sugar and bananas in a large mixing bowl. Stir well until combined. If using an electric mixer, combine using the paddle attachment.
Add egg to the bowl. Mix until well combined, scraping the sides and bottom of mixing bowl as needed.
Add oil and buttermilk. Mix until well combined.
Add the dry ingredients (flour, baking soda, and salt) to the mixing bowl and stir until just combined. Avoid overmixing, as it can result in dense or tough cupcakes.
Optional - stir in any additional ingredients you want such as walnuts, peanuts, chocolate chips, etc...
Divide batter between 6 skillets.
Bake for 18 -20 minutes, or until cakes are fully risen and a toothpick inserted in the center comes out clean.
Place cakes on a rack and allow to cool completely before adding frosting.