Preheat oven to 350 degrees F. Generously grease the bundt pan with cooking spray. Add 2 tablespoons of sugar and twirl the pan around so the sugar will coat the whole pan. Prepare 3 medium size bowl.
In medium bowl #1 , whisk cake flour, baking powder, baking soda and salt and 1 1/2 cup of sugar. This is the dry mixture.
In medium bowl #2, whisk together melted butter, buttermilk, rum, oil, vanilla extract and egg yolks. This is the liquid mixture.
In a clean and oil-free stand mixer bowl, beat the egg whites with the whisk attachment until foamy. Add 1/4 cup of sugar while the mixer is running on low speed. Keep on mixing until it becomes stiff peak. Put this in medium bowl #3.
Put the dry mixture or bowl #1 in the stand mixer with the whisk attachment. Whisk for about 10 seconds.
While the mixer is in low speed, gently add the liquid mixture or bowl #2 into the bowl. Mix until everything is combined. Take the bowl out of the stand.
Add the 1/3 of stiff peak egg whites that is in bowl#3 into the stand mixer bowl. Fold it in until it is combined. Add the rest of the egg whites. Fold until no white streak is showing.
Pour into the greased pan and bake for 40-45 minutes until an inserted toothpick comes out clean. Put on top of a cooling rack.
After about 15 minutes, poke lots of holes into the cake using a skewer. Pour half of the rum sauce into the holes. Wait another 5 minutes and then invert into on to a plate.
Pour the rest of the sauce into a small serving bowl.
Rum Sauce
In a medium saucepan, heat the butter, sugar and water until it boils. Make sure that all the sugar is dissolved.
Take it off the heat, add the rum and gently stir. Let cool before serving.