Make cake mix according to package directions adding vanilla and mixing well.
While mixing cake preheat oven to 350. In 13x9 pan place stick of butter and place into oven to melt.
Take pan out of oven when butter is melted, and spray just the sides with nonstick cooking spray.
On top of butter pour undrained pineapple, sprinkle brown sugar evenly over top of pineapple mixture completely covering mixture (sometimes I use more brown sugar depending on the pineapple juiceness) you want lots of brown sugar and pineapple/butter mixture.
Carefully pour cake mix over the top of the pineapple mixture spreading evenly over the top (don't worry if some of the pineapple mixture comes over the side of the cake mix it will bake fine).
Place in oven and bake according to cake mix directions. Place toothpick in center of cake and if no cake mixture sticks (you will see a little pineapple mixture on toothpick, this is normal) it is done.
Let cool in pan. When cool cut cake, flipping upside down so pineapple mixture is ontop when serving. Enjoy.