Preheat oven to 350°F and coat a 9x13 pan with cooking spray along all the bottom and all of the sides.
Place the butter in the bottom of the pan and then into the hot oven until butter melts, approximately 4-5 minutes.
Meanwhile, stir together the brown sugar and salt.
Remove the pan from the oven and swirl so butter covers the bottom. Sprinkle brown sugar mixture over butter, using a spoon to evenly spread into the bottom of the pan. It may not fully absorb and be a little gritty, this is fine.
Arrange pineapple slices over the brown sugar. Place cherries in the center of each slice and around the slices. Set aside.
In a large mixing bowl, stir together the cake mix, pineapple juice (if reserved juices do not equal 1 full cup, add enough water to make 1 cup), eggs, oil and vanilla. Stir well until few lumps remain, about 2 minutes.
Pour cake mix over pineapple slices.
Return to oven and bake for 45-48 minutes, or until it passes the toothpick test.
Remove and immediately run a butter knife along the edges. Place a rectangular serving dish over the top and carefully using hot pads, invert. If the edges get a little too browned, use a serrated knife to trim them off.
Remove the pan and serve warm or at room temperature.
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