Grease the bundt pan with cooking oil spray or butter. Preheat the oven to 350℉.
Whisk the flour, salt and baking powder in a bowl and set aside.
Add the butter, sugar and oil to the stand mixer bowl and mix on medium speed for 5 minutes until light and fluffy. Stop the mixer and scrape the sides with a spatula.
Add the eggs one at a time and keep beating until all the eggs come in. Add the vanilla extract and mix until well incorporated.
On low speed, add the buttermilk alternating with the flour mixture till well combined. Stop the mixer and scrape the sides again.
Add the rum and mix well until combined.
Transfer the cake batter to the greased cake pan.
Bake at 350℉ in the middle rack of the oven for 60 minutes until a toothpick or cake tester inserted in the center comes out clean.
Remove the cake from the oven and let it sit in the baking pan for 10 minutes. Transfer to a cooling rack and let it cool.
While the cake is cooling, make the glaze.
In a pot over medium heat, melt the butter, add the rum, sugar, water and heavy cream, and stir until combined.
Boil for 3-5 minutes, then transfer to a bowl and let it cool.
When the cake and rum sauce are cooled, pour the sauce all over the cake and serve immediately. Make sure the cake is well soaked with the sauce.