Put the chopped pineapple in a baking dish, cover with foil and roast in the oven until soft and lightly golden brown. Leave the oven at 350 degrees F.
Put the sugar, vanilla bean and 1/2 cup of water in a medium saucepan and cook over high heat until it turns a light caramel. Add the chopped pineapple, cream, salt and arbol and cook until slightly reduced. Let cool for about 5 minutes before adding it to a blender. Remove the vanilla bean, then carefully transfer the mixture to a blender and blend until smooth. Strain through a medium mesh strainer into a bowl. Return the mixture to the saucepan and reduce it to a sauce consistency. Pour the caramel into a seasoned 10-inch cast iron pan and let cool slightly. Arrange the pineapple slices on top of the caramel and set aside.
For the cake:
Cream the butter and sugar in the bowl of a stand mixer fitted with paddle attachment until light and fluffy, scraping down the sides and bottom of the bowl once. Add the eggs, 1 at a time, beating until each is incorporated.
Whisk together the flour, cornmeal, baking soda, baking powder and salt in a bowl until combined. Whisk together the buttermilk, sour cream, rum, vanilla extract and vanilla seeds in a measuring cup. Alternate the dry and wet ingredients starting and ending with the flour and mix until just combined, scraping the sides and bottom of the bowl. Pour the batter over the pineapple in the cast iron pan and bake until the top is lightly golden brown and a toothpick inserted into the center comes out with a few moist crumbs attached, 30 to 40 minutes. Cool on a baking rack for 10 minutes, then invert onto a large platter. Let cool for 30 minutes before serving.