Preheat oven to 350 degrees F. Drain the pineapple slices (reserve ½ cup juice for cake) and maraschino cherries.
Pour the melted butter into the bottom of a 9 X 13 inch baking dish. Sprinkle the brown sugar evenly over the butter.
Place the pineapple slices over the bottom of the pan evenly. Place maraschino cherries in middle of pineapple slices, and in spaces around edges of the pan.
In a stand mixer with whisk attachment, combine vanilla cake mix, instant vanilla pudding mix, eggs, milk, vegetable oil, sour cream, and pineapple juice. Beat on low speed for 30 seconds, then medium speed for 1 ½ to 2 minutes, scraping the sides of the bowl as needed. Pour cake batter into baking dish over the pineapple and cherries, being careful not to move them. Smooth batter with rubber spatula.
Bake for 40-45 minutes, until inserted toothpick comes out clean. Let cool for 5 minutes, then invert by placing a rectangular platter or baking sheet over the top of the cake pan, holding the sides of both the pan and platter firmly, and flipping over. Remove the cake pan gently. Serve immediately or chilled, optional topping whipped cream.