Set oven to 350°F (or 325°F for a dark/nonstick pan).
Pour melted butter evenly into the bottom of your greased 13x9-inch pan.
Sprinkle brown sugar over the butter.
Arrange pineapple slices in rows.
Add maraschino cherries to the centers of the slices if using.
In a large mixing bowl, combine the cake mix, eggs, oil, and 1 cup of reserved pineapple juice instead of water.
Beat on low for 30 seconds, then on medium speed for 2 minutes, scraping the bowl occasionally.
Gently pour the batter over the arranged fruit and spread evenly.
Bake for 40–45 minutes or until a toothpick inserted in the center comes out clean.
Let the cake cool in the pan for 10 minutes.
Loosen the sides with a knife.
Place a tray or cutting board on top of the cake and carefully flip it over.
Let it sit for a minute before lifting off the pan slowly.