Preheat the oven to 350°F. Generously grease a 10 to 12 cup Bundt pan with butter, dust with flour and tap out the excess, then sprinkle the pecans evenly into the bottom.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
In a large bowl, whisk the buttermilk, sour cream, rum and sugar until smooth. Whisk in the melted butter, oil and vanilla.
Add the dry ingredients to the wet mixture and fold just until combined. Pour the batter over the pecans, smooth the top and tap the pan on the counter to release air bubbles.
Bake for 50-55 minutes, until a toothpick inserted into the thickest part comes out clean. Cool in the pan for 10 minutes.
While the cake cools, make the glaze. Melt the butter in a saucepan, stir in the water and sugar and boil for 5 minutes, stirring constantly.Remove from the heat and carefully stir in the rum, as it will bubble up.
Invert the warm cake onto a rimmed serving plate and poke holes all over the top and sides with a skewer. Slowly spoon the warm glaze over the cake, letting it soak in before adding more.Let the cake rest for at least 1 hour so the glaze fully absorbs before slicing.