Preheat oven to 325F. Spray a (10- to 15-cup) Bundt pan with baking spray or butter and flour it.
Combine flour, salt, and baking powder in a large bowl then whisk together.
In the bowl of a stand mixer, beat butter, oil, and sugar, on medium speed until light and fluffy, about 5 minutes. Add eggs, one at a time, beating until combined after each addition. Add vanilla, beating just until combined.
With the mixer on low, add flour mixture alternating with rum and buttermilk, beginning and ending with flour mixture just until combined.
Pour batter into pan. Gently tap on counter a few times to settle batter.
Bake for 65 minutes or until a wooden pick inserted in center comes out clean. Let cool in pan for 15 minutes. Invert onto a cool rack and let cool completely. Glaze cooled cake with half of Buttered Rum Sauce. Serve with remaining sauce.