Preheat oven to 400°F.
Mix 1 cup of white sugar, 1 cup of light brown sugar and ¼ teaspoon cinnamon together. Set aside.
Melt 4 tablespoons of butter in a 10-inch cast-iron skillet over medium heat with a pinch of salt and ⅛ cup light brown sugar. Cook until melted and thick. Remove from heat.
For the cake, mix ¼ cup butter, coconut oil, vanilla and milk together until smooth in a standing mixer. Add eggs. Blend. Add flour, baking powder, cinnamon and 1¾ cups sugar mixture. Blend until smooth. Set aside.
In bottom of buttery cast-iron skillet, place sliced bananas, making sure they don’t overlap.*
Pour cake batter gently over bananas. Sprinkle top with ¼ cup remaining cinnamon sugar mixture.
Cook 20 minutes at 400°F. Reduce heat to 350°F and cook approximately 10 additional minutes, checking cake occasionally to make sure it isn’t burning and is cooking through. A toothpick inserted in the center that comes out clean will tell you it's done.
Serve warm with maple or vanilla ice cream and coffee.
*Banana preservation tip: If you worry about your bananas turning brown in your dessert (mostly for aesthetics as it won't hurt the taste), you can gently toss them with a heavy squeeze of lemon juice before lining them in the bottom of the skillet.