Butter the bottom and sides of the cake pan and line the bottom with parchment paper.
Cut the pineapple. If you have a pineapple corer, use that then slice the pineapple rounds into quarter circle pieces. The pineapple slices should be as thin as possible to ensure complete coverage of the bottom of the pan but if there are some gaps, its not the end of the world.
In a large saucepan, add the pineapple juice, brown sugar and rum. Stir over medium heat until the sugar is dissolved then add the pineapple slices.
Add water to the saucepan so that the pineapple is barely submerged. Bring the mixture to a gentle simmer and cook until the pineapple is soft and translucent. Transfer the pineapple slices to a plate, reserving the juices in the pan.
Bring the mixture in the saucepan back to medium heat and add the butter and a pinch of salt. Cook until the butter is melted and the mixture is thick and bubbling, around 5-7 minutes. Swirl the pan often to ensure even heating.
Pour the caramel sauce into the prepared cake pan and tilt the cake pan to ensure even coating. Once the caramel has cooled, set the pineapple on the bottom of the pan, creating an overlapping pattern. Set the pan aside while the cake batter is prepared
Preheat the oven to 350 and place the pecans on a lined baking tray. Bake for ten minutes or until toasted and fragrant. Shake the pan halfway through toasting. Set the toasted pecans aside to cool.
Combine the flour, baking soda, baking powder, cinnamon, salt, and nutmeg in the bowl of the food processor. Add the cooled pecans and process the mixture in long pulses until the pecans are finely ground. Set the flour mixture aside.
In a stand mixer with the paddle attachment, beat the butter, granulated sugar and brown sugar until light and fluffy. Scrape down the sides once or twice while beating the butter mixture.
Add the eggs to the butter mixture one at a time, mixing thoroughly between each addition. Add the vanilla extract and mix thoroughly.
With the mixer on low speed, add a third of the dry ingredients and incorporate into the mixture. Add half of the buttermilk and mix to incorporate. Repeat with another third of the dry ingredients, the second half of the buttermilk and finally the last third of the dry ingredients.
Turn off the mixer and fold the batter a few times to ensure all the ingredients are fully incorporated. Put the batter into the cake pan on top of the pineapple in small dollops. Use a spatula to smooth the batter on top into an even layer.
Bake the cake in the center rack of the oven on 350 degrees Fahrenheit for 40-45 minutes. The cake is done when a toothpick inserted into the center comes out clean and the cake is pulling away from the sides of the pan a little bit.
Let the cake cool on a wire rack for 15 minutes. Using a paring knife, cut around the edge of the pan to help release the cake from the pan. Invert the cake onto a plate or serving platter and slowly remove the pan. Let the cake cool completely and serve.