Step 1

Adjust oven rack to middle position and heat oven to 350 degrees.

Step 2

Grease a bundt pan liberally with cooking spray.  Then add a few tablespoons of granulated sugar to the inside of the pan, and gently tap and turn it around until every square inch of the inside of the pan is coated in sugar.  (Be especially sure that the inner cone of the bundt pan is coated in sugar so that the cake does not stick.)  Set aside.

Step 3

In a large bowl, whisk together cake flour (*or see substitute below), baking powder, salt, baking soda, and 1 1/2 cups sugar until combined.  In a different medium bowl or large measuring cup, whisk together melted butter, buttermilk, rum, oil, vanilla, and yolks until combined.  Set aside.

Step 4

In clean bowl of a stand mixer fitted with whisk attachment, beat egg whites at medium-high speed until foamy, about 30-45 seconds.  With whisk still running, add in the remaining 1/4 cup sugar.  Continue to beat until stiff peaks just form, about 60-90 more seconds. Transfer the whipped egg whites to a separate bowl and set aside.

Step 5

Add the flour mixture to now-empty mixing bowl. Using the whisk attachment again, gradually pour in butter mixture and mix on medium-low speed until it is just combined, about 15 seconds. (Don’t overmix!)  Stop the mixer and scrape whisk and sides of bowl. Return mixer to medium-low speed and beat until the mixture is smooth and fully incorporated, about 10 to 15 more seconds.  Fold about 1/3 of the whipped egg whites into batter until just combined, then add the remaining whites and gently fold in until no streaks remain.

Step 6

Pour the batter evenly into the sugared bundt pan.  Bake for 40-50 minutes, or until a toothpick inserted into the middle of the cake comes out clean.  (Don’t overbake!)  Transfer the cake to a cooling rack and let it cool for at least 15 minutes.  (While the cake cools — or even better, while it’s in the oven — make the butter-rum sauce.  Instructions below.)

Step 7

Once the cake is slightly cool, use a toothpick or skewer to poke lots of holes all over the surface of the cake (while it is still in the pan.)  Pour about half of the butter rum sauce over the surface of the cake.  Let it rest for 5-10 minutes until the sauce is absorbed.  Then carefully invert the cake onto a serving platter or pedestal, and drizzle the remaining sauce evenly on top of the cake.

Step 8

Serve immediately, or cover until ready to serve.

Step 9

How To Make The Butter-Rum Sauce:

Step 10

In a medium saucepan, whisk together sugar, butter and water until combined.  Bring to a boil over medium-high heat, stirring occasionally, and continue to boil for 5 minutes or until the sugar is completely dissolved.  Remove from heat and gradually stir in rum (carefully, as the rum will make the sauce boil up again).