For the caramel: Preheat the oven to 350 degrees F. Butter a 9-by-13-inch baking dish and set aside.
Melt the 4 ounces butter in a medium skillet over medium heat. Add the brown sugar and vanilla bean and seeds and cook, stirring, until smooth, 3 to 5 minutes. Carefully remove the vanilla bean and pour the caramel mixture into the prepared baking dish. Shingle the pineapple slices in 2 rows on top of the hot caramel down the length of the dish. Set aside.
For the cake: Sift together the flour, baking powder and salt in a medium bowl and set aside. Cream the granulated sugar and butter in the bowl of a stand mixer on medium speed until light and fluffy. Add the sour cream and continue to mix to combine. Add the vanilla extract and egg and continue mixing. Add the flour mixture and milk in alternating additions, starting and ending with the flour.
Spread the batter evenly over the prepared pineapple dish and bake until a cake tester inserted into the center comes out clean with a moist crumb, about 45 minutes. Allow to cool on a rack 5 to 10 minutes. Run a knife around the edge of the baking dish, place a serving plate on top and carefully flip it over. Slowly lift off the baking dish, being careful of steam, and replace any fruit that may have stuck to the dish. Serve.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.