Pour ½ of melted butter into cast iron skillet or round cake pan. Swirl the pan until the bottom and sides are well-coated with butter to prevent the cake from sticking. Sprinkle brown sugar evenly over the melted butter in the skillet or cake pan. Place pineapple slices on top of the brown sugar. Add maraschino cherries to the center of each pineapple slice.
In a separate bowl, combine buttermilk, the remainder of your melted butter, eggs, sugar, flour, baking powder, and salt. Whisk together until well-combined. Pour evenly over the pineapple, cherry, and brown sugar layer in the skillet or cake pan.
Place in the oven and bake for approximately 45 minutes to 1 hour or until toothpick or skewer inserted in the center comes out clean.
Remove the cake from the oven and allow to stand for about 5-10 minutes before transferring to a cake stand.
When ready to transfer, place cake stand on top of skillet, then carefully flip to where the pineapple upside down cake comes out onto the cake stand.