Preheat oven to 350°F. Prepare a 9x13-inch pan with cooking spray. Pour melted butter in pan and sprinkle with brown sugar. Arrange pineapple slices on top of the brown sugar, wiht a cherry in each pineapple ring.
In a large bowl, add the cake mix, water, oil, eggs, and vanilla extract and beat until moistened, 30 seconds. Beat on medium speed for 2 minutes. Pour the batter over pineapple and cherries.
Bake 30 to 35 minutes or until toothpick inserted in center comes out clean.
Cool cake on wire rack for 5 minutes. Next, invert the cake onto a serving platter. Drizzle any remaining pan juices over cake, using a spatula.
Cut into squares and serve whipped topping. Serve the cake warm or cool.