Preheat oven to 180ºC. Pour the melted butter into the bottom of a 33 x 23cm cake pan. Sprinkle brown sugar evenly over butter. Arrange pineapple slices on top of brown sugar. Place a cherry in the center of each slice and arrange remaining cherries around the slices. Press gently into brown sugar.
Add enough water to reserved pineapple juice to measure 230ml. Beat cake mix, eggs, oil and pineapple juice mixture in a large bowl on low speed until moistened. Beat on medium speed for 2 minutes. Pour the cake mixture over the pineapple and cherries and gently level the surface with a spatula.
Bake 42 to 46 minutes until golden and toothpick inserted in center of cake comes out clean. Immediately run knife around sides of the pan to loosen cake.
Place a heatproof serving plate or board upside down on the on top of the cake pan. Turn plate and pan over together. Leave pan over cake for 5 minutes so the brown sugar topping can drizzle over cake; remove pan.
Beat whipping cream mix and cold milk in medium bowl as directed on package. Serve the warm cake cut into squares with a spoonful of whipping cream.