Preheat the oven to 350 degrees F (175 degrees C).
Put 1/2 cup butter into a 9×13-inch cake pan and heat in the preheated oven until melted. Remove from oven and brush sides of pan with some of the melted butter.
Evenly sprinkle brown sugar over the melted butter in the pan. Place pineapple rings on top of the sugar-butter layer. Place maraschino cherries in the middle of the pineapple rings and decorate in between as desired. Reserve 1/2 cup crushed pineapple and fill the gaps with remaining crushed pineapple to completely cover the pan. Drain the reserved crushed pineapple very well.
Combine cake mix, 1 cup pineapple juice, eggs, and 1/2 cup melted butter in a large bowl; beat using an electric mixer on low speed until moistened. Increase speed to medium and beat for 2 minutes. Mix in rum and the reserved 1/2 cup crushed pineapple. Stir to thoroughly combine. Pour batter into the prepared pan and smooth out with a spatula.
Bake in the preheated oven until a toothpick inserted in the middle comes out clean, making sure not to push all the way through the bottom of the cake, 30 to 35 minutes.
Remove cake from the oven and immediately run a knife around the edge of the pan to thoroughly loosen it from the sides. Let cool in pan for a few minutes. Place serving platter upside-down over the pan while cake is still warm and, holding both together, flip over and leave pan on for a couple minutes. Slowly and carefully lift cake pan from cake.
If you get less than 1 cup of pineapple juice from the can, make up the difference with water.
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