Preheat your oven to 350°F (175°C). Place a 10-inch cast iron skillet over medium heat and melt the butter. Once melted, add the brown sugar and stir until it dissolves and the mixture starts bubbling.
Carefully arrange the sliced bananas in a single layer over the caramel sauce in the skillet. Set aside.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. In a separate bowl, beat together the granulated sugar, vegetable oil, eggs, and vanilla extract until well combined.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients, mixing until just combined. Be careful not to overmix, as this can result in a dense cake.
Pour the cake batter over the banana layer in the skillet, spreading it evenly to cover the bananas. Smooth the top with a spatula to ensure an even surface.
Place the skillet in the preheated oven and bake for 35-40 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Once the cake is baked, remove it from the oven and let it cool in the skillet for about 10 minutes. Carefully invert the skillet onto a serving plate, allowing the cake to release from the skillet and revealing the beautiful caramelized banana topping.
Serve the Banana Upside Down Skillet Cake warm or at room temperature. It pairs perfectly with a scoop of vanilla ice cream or a dollop of whipped cream.