To Make The Cake:
Start by preheating the oven to 350 degrees and spraying a 12-cup bundt pan with a nonstick spray or grease pan (make sure you really prepare your pan well to ensure no sticking for this cake).
In the bowl of a stand mixer fitted with the whisk attachment, beat the butter for 2 minutes on high speed.
Slowly add the granulated sugar. Mix together for an additional 5 minutes, until very pale yellow and fluffy.
Add the eggs 1 at a time, combining well after each addition and scraping down the sides and bottom of the bowl as needed.
Turn your mixer down to its lowest speed and slowly add the flour in 2 batches. Add the salt. Be careful not to overbeat. Add the sour cream, crushed pineapple, and vanilla extract. Scrape down the sides and bottom of the bowl and mix the batter until just combined. Be careful not to overmix.
To Assemble with top of the cake:
To assemble the cake, pour melted butter in the bottom of the sprayed pan then sprinkle the butter with brown sugar.
Line the bottom of the pan with the pineapple rings and add 1 cherry to the center of each pineapple ring.
Evenly pour cake batter over pineapples and bake for 70-80 minutes or until a toothpick inserted into the center of the cake comes out mostly clean but still moist.
Allow cake to rest for 10 minutes then quickly invert cake onto a serving plate.
For the Rum Syrup:
In a small pot set over medium heat, combine all the ingredients and cook over medium-high heat.
Reduce the mixture by about a third and thickened it, for about 5 minutes. Remove from heat and let cool for 10 minutes.
Finally, poke sides and top of the cake and pour syrup over the cake and allow to seep in then cool.