For the cake: Preheat the oven to 325 degrees F. Spray a 10-by-2-inch round baking pan well with nonstick spray.
Cream together the sugar, clarified butter and salt in a stand mixer fitted with the paddle attachment. When the mixture is sandy and pasty, add the eggs and vanilla; the mixture will appear almost broken. In a separate bowl, sift together the flour, baking powder and baking soda. Gradually mix the dry ingredients into the batter. Finish by adding the buttermilk, making sure there are no lumps. Cover and set aside.
Peel the pineapple and cut crosswise into 1/4-inch disks. Use a ring cutter to punch the core out of each ring.
To make the caramel: In a pot fitted with a candy thermometer, mix the sugar and water to make a 'wet sand,' making sure the walls of the pot are clean. Bring the mixture up to 298 degrees F or until the sugar begins to turn light brown. Then hit it with the rum, cream, vanilla, salt and butter. Pour the hot caramel into the bottom of the baking pan, then evenly line it with the pineapple slices. Pour cake batter over the top.
Bake until a cake tester comes out clean and the top springs back when pressed, 20 to 25 minutes.
While the cake is baking, make the whipped cream: Place the cream, sugar, vanilla and salt in a stand mixer fitted with the whisk attachment. Whisk on speed 2 until soft, shiny peaks form, about 5 minutes. Set aside.
To make the brittle: Line a baking sheet with parchment paper and spray well with nonstick spray. In a large pot, mix the sugar, water and corn syrup together. On medium heat bring sugar mixture up to 264 degrees F. Then add your macadamia nuts and your salt. Return to heat folding gently until the mixture reaches 315 degrees F. Then fold in your baking soda butter and vanilla extract and pour the mixture out carefully onto your greased sheet pan. As it is cooling you need to be spreading it to your desired width.
When the cake comes out. Let it sit for 10 minutes and then turn it upside down and dump it in one fluid motion onto a cutting board. Cut into about 12 pieces top with a dollop of whipped cream and some of the cooled brittle.