Preheat oven to 350°F. Place the butter in a 13x9-inch metal cake pan. Place the pan in the oven for about 5 minutes to melt the butter.

Make sure the melted butter is evenly distributed over the bottom of the pan. Sprinkle the brown sugar evenly over the top.

Drain the pineapple slices, reserving ½ cup of the juice. Arrange the pineapple slices on top of the brown sugar in the pan. Place a cherry in the center of each pineapple slice; arrange the remaining cherries around the edges and between the slices as desired. Set the pan aside.

In a large bowl, combine the cake mix, pudding mix, sour cream, vegetable oil, eggs, vanilla, and reserved pineapple juice. Beat with an electric mixer at low speed for about 30 seconds, then increase speed to medium and beat for 2 minutes.

Pour the batter evenly over the pineapple slices in the pan. Bake for 42-45 minutes, or until the cake is golden brown and a toothpick inserted in the center comes out clean.

Remove the cake from the oven. Immediately run a knife around the sides of the pan to loosen the cake. Place a large platter upside-down over the top of the cake; carefully (using oven mitts) grab the platter and the cake pan and invert so the platter is on the bottom and the cake pan is on the top.

Let the cake rest with the pan on top of it for 5 minutes to allow the syrup to drip over the cake. Remove the pan and let the cake cool for 30 minutes.

Serve with whipped cream or Cool Whip.