Pre-heat oven to 350℉.
Make cake batter. In a large bowl, or large bowl on stand mixer, bring together softened butter, brown sugar, sour cream, milk, egg, vanilla extract, and pineapple juice. Whisk to combine. Add flour, baking powder, baking soda, and salt to the mixture. Fold to combine.
Make topping. Pat dry pineapple rings and maraschino cherries, and set aside. Heat cast iron skillet and add butter. Once butter melts, switch off heat, and sprinkle brown sugar onto melted butter. Immediately add pineapple slices and maraschino cherries.
Pour cake batter onto pineapple rings and cherries. Use a spatula to evenly spread out cake batter.
Bake at 350℉ for 40 minutes.
Remove pineapple upside down cake from oven. Let it cool for 20-30 minutes.
Place a large plate onto cast iron skillet, then gently flip cake over with plate in the bottom.
Serve and enjoy!