Preheat oven to 350°F. Place the butter in a 9 x 13-inch metal baking pan. Transfer the pan to the oven until the butter melts, about 4 minutes. Tilt the pan so that the butter evenly coats the bottom. Spray the sides of the pan with nonstick spray.
Sprinkle the brown sugar evenly over the melted butter, making sure it covers the entire surface.
Arrange 8 pineapple slices on top of the brown sugar. Place a cherry in the center of each pineapple slice. Scatter extra cherries in the gaps between the rings if desired.
Prepare the cake batter according to the package directions, but replace the water with the reserved pineapple juice. If you don't have enough pineapple juice to total the required amount of water, just add a little bit of water to reach the correct amount. For instance, most cans contain about ¾ cup of juice, so add an extra ¼ cup of water to equal the 1 cup called for on the box.
Pour the cake batter gently over the pineapple layer. Try not to disturb the fruit arrangement.
Bake for 25 to 30 minutes, or until the top is golden and a toothpick inserted in the center comes out clean. The edges will be slightly browned and may be pulling away from the pan.
Let the cake cool in the pan for about 5 minutes (no longer than 10). Run a knife along all four edges of the pan. Place a serving platter face-down over the pan, hold both firmly, and flip. Leave the pan on top for 1 to 2 minutes so the caramel settles, then carefully lift straight up.
Let cool for at least 30 minutes. Slice and serve warm, or cool completely. Top with whipped cream, vanilla ice cream, and/or caramel sauce if desired.