Melt 4 tablespoons of the butter in a 10-inch cast iron skillet, over medium heat. Add the brown sugar and stir to combine. Increase the heat to medium high and cook until the sugar mixture is bubbly, about 2 minutes. Arrange fruit slices in the skillet in a pretty pattern and continue to cook for 2 minutes, or until the sugar mixture turns an amber color. Watch this! So easy to burn and stink up the whole house with burned sugar. Sprinkle on about 1 tsp of the cinnamon and grate about half of the nutmeg nut. Remove the pan from the heat. Cover loosely with lid. (The mixture will continue to cook even though the heat is off.)
Preheat the oven to 375 degrees F.
Stir together the flours, baking powder, baking soda, remaining cinnamon, nutmeg and salt in a medium mixing bowl.
In the bowl of an electric mixer, cream together the remaining 1/2 cup butter and the granulated sugar until light and fluffy. Add the eggs, 1 at a time, mixing just until incorporated. Really, it's gonna look like it's not mixed...just let it go...less is more! It will make the cake tender if you baby it at this point. Add the flour mixture and buttermilk alternately in 3 batches, mixing at low speed after each addition until just combined. Stir in the vanilla and rum.
Spoon the batter evenly over the pineapple slices in the skillet. Bake in the middle of the oven until golden brown and a tester comes out clean, 35 to 40 minutes.
Cool the cake in the skillet on a wire rack for 4 minutes only. Run a thin knife around the edge of the cake and, wearing oven mitts and working quickly, invert the cake onto a cake plate, keeping plate and skillet firmly pressed together. Carefully lift the skillet off cake and replace any fruit stuck to the bottom of the skillet, if necessary. Viola! and fun to flip!
Some may add some ice cream, whipped cream, or other such dairy non-sense. I recommend pouring a little more rum over it and a big cuppa black coffee.
