Preheat your oven to 350°F (175°C) and grease a nonstick bundt pan with butter or cooking spray.

In a medium saucepan, combine brown sugar and dark rum over medium heat, stirring until boiling. Simmer for 5 minutes until reduced by half. Stir in butter until melted and coat pineapple slices.

Pour the pineapple rum mixture into the bundt pan, ensuring pineapple slices are evenly spread.

In a stand mixer, cream the butter and shortening for 2 minutes, then gradually add granulated sugar and beat until light and fluffy.

Add eggs one at a time, mixing well after each addition, followed by vanilla extract and a splash of dark rum.

Whisk together flour, salt, baking powder, and baking soda in a separate bowl. Gradually add dry ingredients to the wet mixture, alternating with buttermilk, and mix just until combined.

Pour the batter over the pineapple mixture in the bundt pan and evenly distribute.

Bake for approximately 1 hour, checking with a toothpick for doneness.

Let the cake cool in the pan for 10 minutes before inverting onto a serving plate.

Cool completely before slicing and serving, optionally topping with shredded coconut or serving alongside vanilla ice cream.